[GStreamer] don't send transferMode HTTP header
https://bugs.webkit.org/show_bug.cgi?id=136781

Reviewed by Philippe Normand.

No header at all is better than having the wrong header as DLNA
mandates that a missing header has to be tolerated while a wrong
header is an error.

* platform/graphics/gstreamer/WebKitWebSourceGStreamer.cpp:
(webKitWebSrcStart):
* platform/network/HTTPHeaderNames.in:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173614 268f45cc-cd09-0410-ab3c-d52691b4dbfc