rtspconnection: also add Content-Type to HTTP POST request when tunnelling

rtspconnection: also add Content-Type to HTTP POST request when tunnelling

When the GstRTSPConnection class sends a RTSP over HTTP tunnelling
request, the HTTP Content-Type header is missing from the HTTP POST
request.

This isn't a problem with most servers, but there are servers that
rejects the request without there also being a Content-Type header.

RFC 1945:
Any HTTP/1.0 message containing an entity body should include a
Content-Type header field defining the media type of that body.

Apple Dispatch 28:
QuickTime Streaming uses the "application/x-rtsp-tunnelled" MIME
type in both the Content-Type and Accept headers. This reflects
the data type that is expected and delivered by the client and server.

https://bugzilla.gnome.org/show_bug.cgi?id=793110