Redirect response wrapper should commit response

This commit ensures that when using `sendRedirect`, the response wrapper
behaves correctly with regards to the Servlet specification:

1. reset the response buffer to clear any partially written response
2. set the expected response HTTP headers
3. flush the buffer to commit the response

Closes gh-29050

spring-web/src/main/java/org/springframework/web/filter/RelativeRedirectResponseWrapper.java

@@ -16,6 +16,8 @@
 
 package org.springframework.web.filter;
 
+import java.io.IOException;
+
 import javax.servlet.http.HttpServletResponse;
 import javax.servlet.http.HttpServletResponseWrapper;
 
@@ -44,9 +46,11 @@ private RelativeRedirectResponseWrapper(HttpServletResponse response, HttpStatus
 
 
 	@Override
-	public void sendRedirect(String location) {
+	public void sendRedirect(String location) throws IOException {
+		resetBuffer();
 		setStatus(this.redirectStatus.value());
 		setHeader(HttpHeaders.LOCATION, location);
+		flushBuffer();
 	}