mod_dav_svn: Fix SVNCacheTextDeltas and SVNAdvertiseV2Protocol directives 
merging.

* subversion/mod_dav_svn/mod_dav_svn.c
  (create_dir_config): Initialize V2_PROTOCOL and TXDELTA_CACHE to 
   CONF_FLAG_DEFAULT.
  (get_conf_flag): New helper to convert conf_flag enum to boolean.
  (dav_svn__check_httpv2_support, dav_svn__get_txdelta_cache_flag): Use
   get_conf_flag().

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548124 13f79535-47bb-0310-9956-ffa450edef68