ra_serf: Remove DAV: headers from GET requests. This decrease outbound 
traffic during checkout/update by 45%.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__handler_t): Add NO_DAV_HEADERS member.

* subversion/libsvn_ra_serf/update.c
  (fetch_file, svn_ra_serf__get_file): Set NO_DAV_HEADERS=TRUE for GET 
   request.

* subversion/libsvn_ra_serf/util.c
  (setup_serf_req): Add DAV: request headers only if DAV_HEADERS parameter
   is non-zero.
  (setup_request): Pass ! NO_DAV_HEADERS to setup_serf_req() call.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1517472 13f79535-47bb-0310-9956-ffa450edef68