chan_sip: Case insensitive comparison of "defaultuser" parameter.

All the other configuration options are case insensitive, so this one
should be too.

ASTERISK-24355 #close
Reported by: HZMI8gkCvPpom0tM
patches:
  ast.patch uploaded by HZMI8gkCvPpom0tM (License 6658)
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Merged revisions 430993 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 430994 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430995 65c4cc65-6c06-0410-ace0-fbb531ad65f3