Don't send authzid when performing SASL PLAIN

It's not allowed to use the authzid when it's no required according to
RFC 6120 6.3.8:

If the initiating entity does not wish to act on behalf of another
entity, it MUST NOT provide an authorization identity.

Fixes #33
Merge pull request #34 from Flowdalic/issue33

Don't send authzid when performing SASL PLAIN