Revision 1497980

Introduce dynamic detection proxies that cannot handle chunked requests.

* subversion/libsvn_ra_serf/ra_serf.h:
  (svn_ra_serf__probe_proxy): new helper to determine proxy problems

* subversion/libsvn_ra_serf/serf.c:
  (load_config): don't bother clearing USING_CHUNKED_REQUESTS. the
    probe (later) will deal with that.
  (svn_ra_serf__open): if we're potentially using a busted proxy, then
    probe the sucker

* subversion/libsvn_ra_serf/options.c:
  (create_simple_options_body): a body delegate that constructs a
    small OPTIONS request body.
  (svn_ra_serf__probe_proxy): send a small OPTIONS request. toss the
    body. we're only looking for a 411 response.