title: set content-type to utf-8 with angularjs $http
body: <p>I'm uploading a form with files, text etc to the appengine blobstore:</p>
  
  <pre><code>$http({
      method: 'POST',
      url: the_url,
      transformRequest: formDataObject,
      data: event,
      headers: {'Content-Type': undefined}
  })
  </code></pre>
  
  <p>The request sends successfully with the follwing header:</p>
  
  <pre><code>Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryYeRxOO7y3qHNWd83
  </code></pre>
  
  <p>When I try setting the content-type to <code>"multipart/form-data;charset=UTF-8"</code> I lose the boundary and get an error in the response: <code>Bad content type. Please use multipart.</code></p>
  
  <p>What is the correct way to add the UTF-8 charset?</p>