Reset charset field in MockHttpServletResponse

Prior to this commit, calling reset() on MockHttpServletResponse did not
reset the `charset` field to `false` which could result in the
"Content-Type" header containing `;charset=null` which in turn would
result in errors when parsing the "Content-Type" header.

This commit resets the charset field to `false` in
MockHttpServletResponse's reset() method to avoid such errors.

Closes gh-25501

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

@@ -458,4 +458,27 @@ class MockHttpServletResponseTests {
 		assertThat(((MockCookie) cookie).getSameSite()).isEqualTo("Lax");
 	}
 
+	@Test  // gh-25501
+	void resetResetsCharset() {
+		assertThat(response.isCharset()).isFalse();
+		response.setCharacterEncoding("UTF-8");
+		assertThat(response.isCharset()).isTrue();
+		assertThat(response.getCharacterEncoding()).isEqualTo("UTF-8");
+		response.setContentType("text/plain");
+		assertThat(response.getContentType()).isEqualTo("text/plain");
+		String contentTypeHeader = response.getHeader(CONTENT_TYPE);
+		assertThat(contentTypeHeader).isEqualTo("text/plain;charset=UTF-8");
+
+		response.reset();
+
+		assertThat(response.isCharset()).isFalse();
+		// Do not invoke setCharacterEncoding() since that sets the charset flag to true.
+		// response.setCharacterEncoding("UTF-8");
+		response.setContentType("text/plain");
+		assertThat(response.isCharset()).isFalse(); // should still be false
+		assertThat(response.getContentType()).isEqualTo("text/plain");
+		contentTypeHeader = response.getHeader(CONTENT_TYPE);
+		assertThat(contentTypeHeader).isEqualTo("text/plain");
+	}
+
 }