Revert b198927; always attempt to parse JSON

This way if there is a PHP error which outputs a 200 OK text/html response, Flarum will correctly show an error message.

framework/core/js/lib/App.js

@@ -227,13 +227,11 @@ export default class App {
         throw new RequestError(status, responseText, options, xhr);
       }
 
-      if (xhr.getResponseHeader('content-type').indexOf('json') !== -1) {
       try {
         return JSON.parse(responseText);
       } catch (e) {
         throw new RequestError(500, responseText, options, xhr);
       }
-      }
     };
 
     if (this.requestError) this.alerts.dismiss(this.requestError.alert);