ra_serf: Improve memory usage in commit processing.

* subversion/libsvn_ra_serf/commit.c
  (close_file): Use scratch_pool for PROPPATCH request.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520028 13f79535-47bb-0310-9956-ffa450edef68