Move the setting of SVN_AUTH_PARAM_SERVER_GROUP from libsvn_ra_serf
to libsvn_ra so that it applies to all RA layers.  This fixes issue
4512: config server groups should work with the svn:// protocol.

* subversion/libsvn_ra/ra_loader.c
  (svn_ra_open4): Set SVN_AUTH_PARAM_SERVER_GROUP.

* subversion/libsvn_ra_serf/serf.c
  (load_config): No need to find group or set SVN_AUTH_PARAM_SERVER_GROUP.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1609499 13f79535-47bb-0310-9956-ffa450edef68