rtsp-client: RTP Info exists conditionally in PLAY; possible issue with RECORD and onvif backchannel
If RTP Info is missing and it is not a receiver only, eg. audio backchannel. Then return GST_RTSP_STS_INTERNAL_SERVER_ERROR. In rfc2326 it says RTP-info is req. but in RFC7826 it is conditional.
Since 1.14 there is audio backchannel support. Thus RTP-info is conditional now. So when audio backchannel only mode, there is no RTP-info.
Fixes #82 (closed)