Merge branch '6.4' into 7.0

* 6.4:
  send necessary HTTP headers

Author: javiereguiluz

http_client.rst

@@ -662,6 +662,7 @@ when the streams are large)::
     $client->request('POST', 'https://...', [
         // ...
         'body' => $formData->bodyToString(),
+        'headers' => $formData->getPreparedHeaders()->toArray(),
     ]);
 
 If you need to add a custom HTTP header to the upload, you can do::