Expose protected method in AbstractJackson2HttpMessageConverter

Closes gh-25509

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -284,7 +284,15 @@ private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) th
 		}
 	}
 
-	private static Charset getCharset(@Nullable MediaType contentType) {
+	/**
+	 * Return the charset to use for JSON input.
+	 * <p>By default this is either the charset from the input {@code MediaType}
+	 * or otherwise falling back on {@code UTF-8}.
+	 * @param contentType the content type of the HTTP input message
+	 * @return the charset to use
+	 * @since 5.1.18
+	 */
+	protected Charset getCharset(@Nullable MediaType contentType) {
 		if (contentType != null && contentType.getCharset() != null) {
 			return contentType.getCharset();
 		}