Add `X-Requested-With: XMLHttpRequest` header so Rails can recognize XHR

Backport from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3732

app/assets/javascripts/lib/utils/axios_utils.js

@@ -2,6 +2,8 @@ import axios from 'axios';
 import csrf from './csrf';
 
 axios.defaults.headers.common[csrf.headerKey] = csrf.token;
+// Used by Rails to check if it is a valid XHR request
+axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';
 
 // Maintain a global counter for active requests
 // see: spec/support/wait_for_requests.rb