* subversion/mod_dav_svn/repos.c   (dav_svn_prep_working): For attempted unsupported operations like     multi-author commits, return a 501 (Not implemented) rather than     a 500 (Internal server error).
* subversion/mod_dav_svn/repos.c
  (dav_svn_prep_working): For attempted unsupported operations like
    multi-author commits, return a 501 (Not implemented) rather than
    a 500 (Internal server error).

Review by: Sander Striker <striker@apache.org>

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