FreeSWITCH-FS-6836

Merge pull request #640 in FS/freeswitch from ~VIPKILLA/freeswitch-eschmidbauer:FS-8668 to master

* commit 'f2fcfe1a75833cf0c5e9b599e12169674460351a':
  FS-8668: allow channel variable prefix-a-leg to override global setting in mod_xml_cdr