ra_serf: fix a lot of merge_tests.
ra_serf: fix a lot of merge_tests.

Situation: When requesting the mergeinfo from the server, the request body is 
provided in a series of buckets to serf. Problem is, when authentication is 
enabled svn might receive a 401 result on the mergeinfo request. In this case 
ra_serf will try the request again with correct authentication info, but, the
original request body is now already consumed (and destroyed).

Solution: use serf's body_delegate mechanism to create the request body on 
demand, and if needed twice.

* subversion/libsvn_ra_serf/mergeinfo.c
  (mergeinfo_context_t): add extra fields required to create the mergeinfo 
   request body.
  (create_mergeinfo_body): new function that creates the body of the mergeinfo
   REPORT request on demand. The body creation code was copied from ...
  (svn_ra_serf__get_mergeinfo): ... here. Setup the body delegate and pass the
   info to serf.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion