title: change subject of formspree with ajax
body: <p>I'm using formspree within an angular app. Everything is working except I can't seem to change the subject of the email. </p>
  
  <p>Html... </p>
  
  <pre><code>&lt;form id="bookingForm" action="https://formspree.io/your@email.com"
    method="POST"&gt;
    &lt;input type="text" name="{{client.name}}"&gt;
    &lt;input type="{{client.email}}" name="_replyto"&gt;
    &lt;input type="hidden" name="_subject" value="Request from {{client.name}}"&gt;
    &lt;!-- some other info --&gt;
    &lt;input type="submit" value="Send"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>JS...</p>
  
  <pre><code>$bookingForm = $('#bookingForm');
   $bookingForm.submit(function(e) {
   e.preventDefault();
   $.ajax({
     url: '//formspree.io/your@email.com',
     method: 'POST',
     data: {
      client: $scope.client.name,
      email: $scope.client.email,
      phone: $scope.client.phone,
      // some other info 
    },
    dataType: 'json',
   });
  });
  </code></pre>
  
  <p>The _replyto seems to be working but _subject is not. </p>