title: Spree AJAX API call 404'ing due to wrong base URL
body: <p>So I'm currently getting 404 errors from dynamic loading fields in Spree on Ruby on Rails. </p>
  
  <p>I've checked my <code>rake routes</code> to ensure that the API route is present, it is. </p>
  
  <p>However I have mounted the Spree engine under a different base URL in config/routes.rb</p>
  
  <p><code>mount Spree::Core::Engine, at: '/shop'</code></p>
  
  <p>So all in all, dynamic loading fields are querying the API with:
   <code>http://&lt;site&gt;/api/v1/states?country_id=232</code>
  instead of <code>http://&lt;site&gt;/shop/api/v1/states?country_id=232</code></p>
  
  <p>Is there an extra route I need to add to my routes.rb file? Or something more sinister?</p>
  
  <p>Thanks.</p>