title: form_tag with remote: true does not make an ajax request
body: <p>I have many different forms in my app, all of them are using <code>remote: true</code>. But one does not work correctly, because is does not use an ajax call.</p>
  
  <p>Cleaned up, it looks like:</p>
  
  <pre><code>&lt;%= form_tag(upload_file_ajax_path, remote: true, multipart: true) do %&gt;
    &lt;%= file_field_tag(:file) %&gt;
    &lt;%= submit_tag("upload") %&gt;
  &lt;%end%&gt;
  </code></pre>
  
  <p>The tag looks like:</p>
  
  <pre><code>&lt;form accept-charset="UTF-8" action="/mycontroller/upload_file_ajax" data-remote="true" enctype="multipart/form-data" method="post"&gt;
    &lt;input name="utf8" type="hidden" value="✓"&gt;
    &lt;input name="authenticity_token" type="hidden" value="1234"&gt;&lt;/div&gt;
    &lt;input id="file" name="file" type="file"&gt;
    &lt;input name="commit" type="submit" value="upload"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>And the routes entry:</p>
  
  <pre><code>post "mycontroller/upload_file_ajax", as: "upload_file_ajax"
  </code></pre>
  
  <p>But checking the call in Chrome Dev Tools, the header says:</p>
  
  <pre><code>Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
  </code></pre>
  
  <p>and not like the other forms look like:</p>
  
  <pre><code>Accept:*/*;q=0.5, text/javascript, application/javascript, application/ecmascript, application/x-ecmascript
  </code></pre>
  
  <p>I added the js files to my layout</p>
  
  <pre><code>&lt;%= stylesheet_link_tag    "application", :media =&gt; "all" %&gt;
  &lt;%= javascript_include_tag "application" %&gt;
  &lt;%= csrf_meta_tags %&gt;
  </code></pre>
  
  <p>And even if I remove all my JS code out of the application.js file (except the <code>//= require jquery</code>...), the form is not working correctly.</p>
  
  <p>What did I miss?</p>