res_pjsip_sdp_rtp: Accept DTLS attributes in top level, not just media session.

#SIPit31
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Merged revisions 424287 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424288 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424289 65c4cc65-6c06-0410-ace0-fbb531ad65f3