Set X-Requested-With for startup JS requests

This is used by Rails to detect if the request is an AJAX request.
We do the same in our axios client.

Changelog: performance

app/views/layouts/_startup_js.html.haml

@@ -8,11 +8,17 @@
 
     if (gl.startup_calls && window.fetch) {
       Object.keys(gl.startup_calls).forEach(apiCall => {
-        // fetch won’t send cookies in older browsers, unless you set the credentials init option.
-        // We set to `same-origin` which is default value in modern browsers.
-        // See https://github.com/whatwg/fetch/pull/585 for more information.
-        gl.startup_calls[apiCall] = {
-          fetchCall: fetch(apiCall, { credentials: 'same-origin' })
+       gl.startup_calls[apiCall] = {
+          fetchCall: fetch(apiCall, {
+            // Emulate XHR for Rails AJAX request checks
+            headers: {
+              'X-Requested-With': 'XMLHttpRequest'
+            },
+            // fetch won’t send cookies in older browsers, unless you set the credentials init option.
+            // We set to `same-origin` which is default value in modern browsers.
+            // See https://github.com/whatwg/fetch/pull/585 for more information.
+            credentials: 'same-origin'
+          })
         };
       });
     }