rtspconnection: Start CSeq at 1

RFC 7826 recommends (but does not require) starting at 0,
but at least one known server implementation fails to copy
request sequence numbers <1 into responses due to an
incorrect null check.

The server known to exhibit this behavior is the Parrot
Streaming Server, serving video from their UAV devices.
A fix has been submitted upstream as well:
https://github.com/Parrot-Developers/librtsp/pull/2

The Parrot developers are known to have tested with LibVLC.
In WireShark debugging, LibVLC appears to start with a CSeq
of 2, which is likely why this bug went unnoticed.

This reverts 487595a7d6, which set this to 0 citing the
RFC. The switch to 0 was thus a recent one; it's therefore
possible server implementors relied on the previous
GStreamer client behavior in their tests as well.

Fixes #624.

gst-libs/gst/rtsp/gstrtspconnection.c

@@ -362,7 +362,8 @@ gst_rtsp_connection_create (const GstRTSPUrl * url, GstRTSPConnection ** conn)
   newconn->url = gst_rtsp_url_copy (url);
   newconn->timer = g_timer_new ();
   newconn->timeout = 60;
-  newconn->cseq = 0;            /* RFC 7826: "it is RECOMMENDED to start at 0." */
+  newconn->cseq = 1;            /* RFC 7826: "it is RECOMMENDED to start at 0.",
+                                   but some servers don't copy values <1 due to bugs. */
 
   newconn->remember_session_id = TRUE;