Allow HTTP authentication without password.

This makes it the plugin useful for any form of webserver-based
authentication (CAS, SAML, etc.); of course, other plugins will
have to be installed to handle connections to the IMAP/SMTP
servers, but at least they don't need to duplicate the basic
HTTP authentication functionality.

**WARNING**: the plugin wasn't tested against latest master, but
a similar version of this patch is in production on a 0.7 installation,
and I don't foresee problems.