Sync MockHttpServletResponse implementations

See gh-33019

Author: sbrannen

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java

@@ -24,6 +24,7 @@
 import java.io.UnsupportedEncodingException;
 import java.io.Writer;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.text.DateFormat;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
@@ -72,6 +73,8 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	private static final TimeZone GMT = TimeZone.getTimeZone("GMT");
 
+	private static final MediaType APPLICATION_PLUS_JSON = new MediaType("application", "*+json");
+
 
 	//---------------------------------------------------------------------
 	// ServletResponse properties
@@ -348,6 +351,10 @@ public void setContentType(@Nullable String contentType) {
 				if (mediaType.getCharset() != null) {
 					setExplicitCharacterEncoding(mediaType.getCharset().name());
 				}
+				else if (mediaType.isCompatibleWith(MediaType.APPLICATION_JSON) ||
+						mediaType.isCompatibleWith(APPLICATION_PLUS_JSON)) {
+						this.characterEncoding = StandardCharsets.UTF_8.name();
+				}
 			}
 			catch (Exception ex) {
 				// Try to get charset value anyway