ra_serf: fix proxy authentication error handling.
ra_serf: fix proxy authentication error handling.

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__credentials_callback): Set maximum limit to the number of
   credentials requests for proxy authentication.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion