Setting the HTML_X_REQUESTED_WITH header in jQuery, which doesn't seem to be set for me unless I force it. If I don't set this, Rails concludes that I'm not making an xhr request and tries to serve HTML, even though the HTTP_ACCEPT header is clearly set to `application/json, text/javascript, */*`.

gistfile1.coffee

$.ajax @api_url,                                                                
  dataType: 'json'                                                              
  beforeSend: (jqXHR, settings) ->                                              
    jqXHR.setRequestHeader('X-Requested-With', 'XMLHttpRequest')                                                                                                          
  success: (data, textStatus, jqXHR) =>                                         
    @onDataLoaded(data, textStatus, jqXHR)                                      
    callback?()                                                                 
  error: (jqXHR, textStatus, errorThrown) =>                                    
    @onLoadFailed(jqXHR, textStatus, errorThrown) 

Another note; what's weird is that jQuery doesn't set this header naturally. Note that if you try and set it by calling setRequestHeader("HTTP_X_REQUESTED_WITH", "XMLHttpRequest") (as it's often documented online) it doesn't work.