[1.6.x] Introduced as_bytes for SafeMIMEText (and other SafeMIME-classes).

This is to provide a consistent interface (namely bytes) for the smtp
backend which after all sends bytes over the wire; encoding with as_string
yields different results since mails as unicode are not really specified.

as_string stays for backwardscompatibilty mostly and some debug outputs.
But keep in mind that the output doesn't match as_bytes!

Backport of 5dfd824d from master.