chan_sip: Decline image streams on unsupported transports

This change allows chan_sip to decline individual image streams over
unsupported transports in the SDP of the 200 response. Previously,
an image stream offer with RTP/AVP as the transport would cause
chan_sip to respond with a 488.

(closes issue ASTERISK-22988)
Reported by: adomjan
Original patch by: adomjan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@406170 f38db490-d61c-443f-a65b-d21fe96a405b