Add carriage return to SMTP handshake

Every header line in SMTP traffic should end with a CRLF (not just a LF) according to http://cr.yp.to/docs/smtplf.html and https://www.ietf.org/rfc/rfc0822.txt.

The fake smtp server now requires a crlf, all tests pass on Rails 4.0, did not try others.

Fyi, I ran into problems when using health_check with AmazonSES SMTP gateway.

Thanks for this awesome gem!