ra_serf: Don't assume length of translated messages.

* subversion/libsvn_ra_serf/util.c
  (append_reason): New function.
  (ssl_server_cert): Use append_reason() instead of blindly adding separators
    and then chopping them off if they aren't needed.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515201 13f79535-47bb-0310-9956-ffa450edef68