Handle stanza ID spoofing

Wonder if @dwd would be kind enough to check over my test cases here in case there's anything I've missed, https://github.com/xmpp-ftw/xmpp-ftw/blob/017c11c7dcaef29102f59d127d138ced2decb72b/test/lib/xmpp.js#L132  (reading the titles should be  enough).

Fixes #62