Update ResponseStatus to conditionally set content type for error handling (#1261)

fixes SITES-28442

Author: bpauli

src/main/archetype/ui.apps/src/main/content/jcr_root/apps/sling/servlet/errorhandler/ResponseStatus.java

@@ -17,11 +17,15 @@
 
 import com.adobe.cq.sightly.WCMUsePojo;
 
+import javax.servlet.RequestDispatcher;
+
 public class ResponseStatus extends WCMUsePojo {
-    
+
     @Override
     public void activate() throws Exception {
         getResponse().setStatus(404);
-        getResponse().setContentType("text/html");
+        if (getRequest().getAttribute(RequestDispatcher.INCLUDE_SERVLET_PATH) == null) {
+            getResponse().setContentType("text/html");
+        }
     }
-}
+}