Don’t expect a charset attribute on a text/css Content-Type

Following the encoding change made in 428a10a7, the character encoding
of a response is no longer forced.

See gh-5459

spring-boot-samples/spring-boot-sample-web-static/src/test/java/sample/web/staticcontent/SampleWebStaticApplicationTests.java

@@ -62,7 +62,7 @@ public class SampleWebStaticApplicationTests {
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
 		assertThat(entity.getBody()).contains("body");
 		assertThat(entity.getHeaders().getContentType())
-				.isEqualTo(MediaType.valueOf("text/css;charset=UTF-8"));
+				.isEqualTo(MediaType.valueOf("text/css"));
 	}
 
 }