Automatically add Rails CSRF tokens into XMLHttpRequest headers.
Load csrf-xhr.js in a <script> anywhere after the <meta name="csrf-token">
that Rails adds to the page.
From now on, any XMLHttpRequest whose destination URL is your local origin
(including both relative URLs as well as URLs which explicitly have the same
origin as the current page) will have an "X-CSRF-Token" token set to the
contents of the <meta name="csrf-token"> that Rails generates in the page's
<head>.
jQuery-ujs does this for you; this does it for you even if you aren't using jQuery.
This was created to decouple elm-rails
from CSRF token management.