rtsp-media: Always use real payloader when creating streams
rtsp-media: Always use real payloader when creating streams

A bin that contains the real payloader might be used as payloader. In this
case we have to get the real payloader for the various properties it provides.

Example use cases for this are bins that payload some media and then have
additional elements that add metadata or RTP extension headers to the stream.

https://bugzilla.gnome.org/show_bug.cgi?id=750800

barisdemiray/gst-rtsp-server,surround-io/gst-rtsp-server,surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server,thaytan/gst-rtsp-server,thaytan/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,barisdemiray/gst-rtsp-server