Fix that global setting wasn't used if a section existed but had no
http-chunked-requests option.

* subversion/libsvn_ra_serf/serf.c
  (load_config): Pass chunked_requests as the default for the
    http-chunked-requests option in the server_group.

Found by: gstein


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/tristate-chunked-request@1502673 13f79535-47bb-0310-9956-ffa450edef68