Use UTF-8 with JSON in MockHttpServletResponse

This commit makes MockHttpServletResponse consistent with the other
parts of the framework where the body of a response is read as an UTF-8
String when the content type is application/json or similar, overriding
the ISO-8859-1 default Servlet encoding.

Closes gh-33019

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java

@@ -30,6 +30,7 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 
+import org.springframework.http.MediaType;
 import org.springframework.web.util.WebUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -620,4 +621,12 @@ class MockHttpServletResponseTests {
 		assertThat(contentTypeHeader).isEqualTo("text/plain");
 	}
 
+	@Test  // gh-33019
+	void contentAsStringEncodingWithJson() throws IOException {
+		String content = "{\"name\": \"Jürgen\"}";
+		response.setContentType(MediaType.APPLICATION_JSON_VALUE);
+		response.getWriter().write(content);
+		assertThat(response.getContentAsString()).isEqualTo(content);
+	}
+
 }