Accept empty Content-Type header

Issue: SPR-12173
(cherry picked from commit 70412a9)

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -331,7 +331,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 */
 	public MediaType getContentType() {
 		String value = getFirst(CONTENT_TYPE);
-		return (value != null ? MediaType.parseMediaType(value) : null);
+		return (StringUtils.hasLength(value) ? MediaType.parseMediaType(value) : null);
 	}
 
 	/**