res_pjsip_sdp_rtp.c: Fix native formats containing formats that were not negotiated.

Outgoing PJSIP calls can result in non-negotiated formats listed in the
channel's native formats if video formats are listed in the endpoint's
configuration.  The resulting call could then use a non-negotiated format
resulting in one way audio.

* Simplified the update of session->req_caps in set_caps().  Why do
something in five steps when only one is needed?

AFS-162 #close

Review: https://reviewboard.asterisk.org/r/4000/
........

Merged revisions 423561 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@423563 f38db490-d61c-443f-a65b-d21fe96a405b