Doesn't handle errors for funky Unicode

[strugee]

Hi! I've recently realized that in 42wim/matterbridge#209 I promised to file a bug upstream (here) but never did. I'm not too sure of the details of what's going on, but it seems that sending fancy Unicode caused this library to generate invalid XMPP stanzas, which then weren't properly error-handled. See the original ticket for details.

/cc @42wim