[#140654507] #10 fix Server.js to give correct output on json and non json response

assets/js/lib/Server.jsx

@@ -44,7 +44,8 @@ export default class Server {
       if (response.status === 204) {
         return response;
       }
-      return response.hasOwnProperty(response) ? response.json() : response;
+      const contentType = response.headers.get("content-type");
+      return contentType && contentType.indexOf("application/json") !== -1 ? response.json() : response.text();
     });
 
     /* istanbul ignore next */