mod_dav_svn: Fix SVNAllowBulkUpdates directive merging.

* subversion/mod_dav_svn/dav_svn.h
  (dav_svn__bulk_upd_conf): Add CONF_BULKUPD_DEFAULT.
* subversion/mod_dav_svn/mod_dav_svn.c
  (create_dir_config): Initialize BULK_UPDATES to CONF_BULKUPD_DEFAULT.
  (dav_svn__get_bulk_updates_flag): Handle CONF_BULKUPD_DEFAULT value.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548105 13f79535-47bb-0310-9956-ffa450edef68