XEP-0198: Don't drop session on failed resume

The 'previd' value provided by the client during a session resume
request includes the client's JID and ejabberd's session ID.  If there
is a session for the requested JID but with a different session ID,
resumption should fail, but that session shouldn't be closed.  This
commit makes sure the latter won't happen.

In practice, this will only make a difference in odd corner cases.
Merge pull request #222 from weiss/keep-session-on-failed-resume

XEP-0198: Don't drop session on failed resume