AUTH not provided in EHLO with SSL/TLS

When using implicit SSL/TLS (so no STARTTLS), the `self._tls_protocol` will never be set. This results in `AUTH` not being present in the EHLO response (given that `auth_require_tls` is true): https://github.com/aio-libs/aiosmtpd/blob/master/aiosmtpd/smtp.py#L833

I would expect that when an SSL/TLS connection is used instead of STARTTLS (and `auth_require_tls` is true), `AUTH` is provided to the client. Currently, we are setting `auth_require_tls` is false to circumvent this issue.