title: Cant pass FormData to fetch's put request
body: <p>So I'm using github/fetch to make xhr calls to my API. Post and get are working fine, but seems like put calls won't pass the FormData properly. </p>
  
  <p>They are passed as form-data in my xhr call. This should be x-www-urlencoded. When I encode my url with the data everything is working fine.</p>
  
  <p>I'm setting the type like this:<br>
  <code>options.headers['Content-Type'] = 'application/x-www-form-urlencoded';</code></p>
  
  <p>What could this be?</p>