Merge r1421380 from trunk:

 * r1421380
   Issue #4272: s/assertion/error/ in the DAV proxy code.
   Justification:
     The assertion has been run into in the wild
   Votes:
     +1: danielsh, rhuijben, philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.7.x@1454652 13f79535-47bb-0310-9956-ffa450edef68