diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
index 7d7be2e752a4..ff4d675ceec3 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -167,11 +167,11 @@ public void setCharacterEncoding(String characterEncoding) {
 
 	private void updateContentTypeHeader() {
 		if (this.contentType != null) {
-			StringBuilder sb = new StringBuilder(this.contentType);
-			if (!this.contentType.toLowerCase().contains(CHARSET_PREFIX) && this.charset) {
-				sb.append(";").append(CHARSET_PREFIX).append(this.characterEncoding);
+			String value = this.contentType;
+			if (this.charset && !this.contentType.toLowerCase().contains(CHARSET_PREFIX)) {
+				value = value + ';' + CHARSET_PREFIX + this.characterEncoding;
 			}
-			doAddHeaderValue(CONTENT_TYPE_HEADER, sb.toString(), true);
+			doAddHeaderValue(CONTENT_TYPE_HEADER, value, true);
 		}
 	}
 
@@ -195,7 +195,8 @@ public PrintWriter getWriter() throws UnsupportedEncodingException {
 		}
 		if (this.writer == null) {
 			Writer targetWriter = (this.characterEncoding != null ?
-					new OutputStreamWriter(this.content, this.characterEncoding) : new OutputStreamWriter(this.content));
+					new OutputStreamWriter(this.content, this.characterEncoding) :
+					new OutputStreamWriter(this.content));
 			this.writer = new ResponsePrintWriter(targetWriter);
 		}
 		return this.writer;
@@ -302,6 +303,7 @@ public boolean isCommitted() {
 	public void reset() {
 		resetBuffer();
 		this.characterEncoding = null;
+		this.charset = false;
 		this.contentLength = 0;
 		this.contentType = null;
 		this.locale = null;
@@ -353,7 +355,7 @@ public boolean containsHeader(String name) {
 
 	/**
 	 * Return the names of all specified headers as a Set of Strings.
-	 * <p>As of Servlet 3.0, this method is also defined HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * @return the {@code Set} of header name {@code Strings}, or an empty {@code Set} if none
 	 */
 	@Override
@@ -364,7 +366,7 @@ public Collection<String> getHeaderNames() {
 	/**
 	 * Return the primary value for the given header as a String, if any.
 	 * Will return the first value in case of multiple values.
-	 * <p>As of Servlet 3.0, this method is also defined in HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * As of Spring 3.1, it returns a stringified value for Servlet 3.0 compatibility.
 	 * Consider using {@link #getHeaderValue(String)} for raw Object access.
 	 * @param name the name of the header
@@ -378,7 +380,7 @@ public String getHeader(String name) {
 
 	/**
 	 * Return all values for the given header as a List of Strings.
-	 * <p>As of Servlet 3.0, this method is also defined in HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * As of Spring 3.1, it returns a List of stringified values for Servlet 3.0 compatibility.
 	 * Consider using {@link #getHeaderValues(String)} for raw Object access.
 	 * @param name the name of the header
diff --git a/spring-web/src/test/java/org/springframework/mock/web/test/MockHttpServletResponse.java b/spring-web/src/test/java/org/springframework/mock/web/test/MockHttpServletResponse.java
index 9d308899dbf5..b207382df444 100644
--- a/spring-web/src/test/java/org/springframework/mock/web/test/MockHttpServletResponse.java
+++ b/spring-web/src/test/java/org/springframework/mock/web/test/MockHttpServletResponse.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -166,11 +166,11 @@ public void setCharacterEncoding(String characterEncoding) {
 
 	private void updateContentTypeHeader() {
 		if (this.contentType != null) {
-			StringBuilder sb = new StringBuilder(this.contentType);
-			if (!this.contentType.toLowerCase().contains(CHARSET_PREFIX) && this.charset) {
-				sb.append(";").append(CHARSET_PREFIX).append(this.characterEncoding);
+			String value = this.contentType;
+			if (this.charset && !this.contentType.toLowerCase().contains(CHARSET_PREFIX)) {
+				value = value + ';' + CHARSET_PREFIX + this.characterEncoding;
 			}
-			doAddHeaderValue(CONTENT_TYPE_HEADER, sb.toString(), true);
+			doAddHeaderValue(CONTENT_TYPE_HEADER, value, true);
 		}
 	}
 
@@ -194,7 +194,8 @@ public PrintWriter getWriter() throws UnsupportedEncodingException {
 		}
 		if (this.writer == null) {
 			Writer targetWriter = (this.characterEncoding != null ?
-					new OutputStreamWriter(this.content, this.characterEncoding) : new OutputStreamWriter(this.content));
+					new OutputStreamWriter(this.content, this.characterEncoding) :
+					new OutputStreamWriter(this.content));
 			this.writer = new ResponsePrintWriter(targetWriter);
 		}
 		return this.writer;
@@ -301,6 +302,7 @@ public boolean isCommitted() {
 	public void reset() {
 		resetBuffer();
 		this.characterEncoding = null;
+		this.charset = false;
 		this.contentLength = 0;
 		this.contentType = null;
 		this.locale = null;
@@ -352,7 +354,7 @@ public boolean containsHeader(String name) {
 
 	/**
 	 * Return the names of all specified headers as a Set of Strings.
-	 * <p>As of Servlet 3.0, this method is also defined HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * @return the {@code Set} of header name {@code Strings}, or an empty {@code Set} if none
 	 */
 	@Override
@@ -363,7 +365,7 @@ public Collection<String> getHeaderNames() {
 	/**
 	 * Return the primary value for the given header as a String, if any.
 	 * Will return the first value in case of multiple values.
-	 * <p>As of Servlet 3.0, this method is also defined in HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * As of Spring 3.1, it returns a stringified value for Servlet 3.0 compatibility.
 	 * Consider using {@link #getHeaderValue(String)} for raw Object access.
 	 * @param name the name of the header
@@ -377,7 +379,7 @@ public String getHeader(String name) {
 
 	/**
 	 * Return all values for the given header as a List of Strings.
-	 * <p>As of Servlet 3.0, this method is also defined in HttpServletResponse.
+	 * <p>As of Servlet 3.0, this method is also defined in {@link HttpServletResponse}.
 	 * As of Spring 3.1, it returns a List of stringified values for Servlet 3.0 compatibility.
 	 * Consider using {@link #getHeaderValues(String)} for raw Object access.
 	 * @param name the name of the header