Fix rebase status check throwing flash message

Fix https://gitlab.com/gitlab-org/gitlab-ee/issues/4473

ee/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue

@@ -57,7 +57,7 @@
       },
       checkRebaseStatus(continuePolling, stopPolling) {
         this.service.poll()
-          .then(res => res.json())
+          .then(res => res.data)
           .then((res) => {
             if (res.rebase_in_progress) {
               continuePolling();

spec/javascripts/vue_mr_widget/components/mr_widget_rebase_spec.js

@@ -88,11 +88,9 @@ describe('Merge request widget rebase component', () => {
           },
           poll() {
             return Promise.resolve({
-              json() {
-                return {
-                  rebase_in_progress: false,
-                  merge_error: null,
-                };
+              data: {
+                rebase_in_progress: false,
+                merge_error: null,
               },
             });
           },