ra_serf: Stringify SVN_AUTH_SSL_OTHER as well

* subversion/libsvn_ra_serf/util.c
  (ssl_server_cert): Handle SVN_AUTH_SSL_OTHER.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515225 13f79535-47bb-0310-9956-ffa450edef68