Set X-CSRF-Token on HTTP requests

https://github.com/emberjs/ember-rails#csrf-token
https://github.com/thoughtbot/ember-cli-rails#csrf-tokens

When `ember-cli-rails` embeds the CSRF meta tags, this initializer
configures the host app to recognize them and set them as outgoing HTTP
headers to play nice with the Rails app's CSRF protection.

app/initializers/csrf.js

@@ -0,0 +1,14 @@
+import Ember from 'ember';
+
+const { $ } = Ember;
+
+export default {
+  name: 'ember-cli-rails-addon:csrf',
+
+  initialize() {
+    $.ajaxPrefilter((options, originalOptions, xhr) => {
+      const token = $('meta[name="csrf-token"]').attr('content');
+      xhr.setRequestHeader('X-CSRF-Token', token);
+    });
+  },
+};