This allow adding rtmpsink after the flv streaming have started. Otherwise,
FLV streamheader is never sent to the server, which cannot figure-out
what is this stream about. It should also help in certain renegotiation
figures. The sink will no longer work without an streamheader in caps,
though there is no known implementation of flvdemux that does not
support this.
https://bugzilla.gnome.org/show_bug.cgi?id=760242