rtmp2: allow NetStream.Play.PublishNotify Message

first of all many thx for that gigantic project and all of your efforts.

i had a couple of incoming RTMP streams (which i couldn't configure or change) that failed to start with rtmp2src. (ffplay, vlc and co worked)

gst-launch-1.0 rtmp://xxxx/pull/yyyyyy
Setting pipeline to PAUSED ...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
ERROR: from element /GstPipeline:pipeline0/GstRtmp2Src:rtmp2src0: Failed to connect
Additional debug info:
../gst/rtmp2/gstrtmp2src.c(966): send_connect_error (): /GstPipeline:pipeline0/GstRtmp2Src:rtmp2src0:
error g-io-error-quark:0: unhandled play result: { "level": "status", "code": "NetStream.Play.PublishNotify", "description": "rtmp://127.0.0.1:3103/live/xxxxxx is now published.", "clientid": 3.65708e+08 }
Execution ended after 0:00:00.841382000
Setting pipeline to NULL ...
Freeing pipeline ...

this MR - fixes it, and makes the rtmp streams that where prev. not working, working fine!

let me know if you'd need me to change something.