rtp: Fix case typo in H263+ mime.

rtp: Fix case typo in H263+ mime.

http://tools.ietf.org/html/rfc3555#section-4.2.6 says the canonical
mime subtype is "H263-1998", not "h263-1998". Original code was added
in r183101 on 2009-03-19 02:26:50 +0100.

This fixes issues with Polycom phones.

ASTERISK-23665 #close
ASTERISK-23665 #comment Patch r3529.patch uploaded by Guillaume Maudoux, backported by me.
Review: https://reviewboard.asterisk.org/r/3529/
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Merged revisions 413787 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 413788 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@413789 65c4cc65-6c06-0410-ace0-fbb531ad65f3