Avoid Interceptor instance objects

I had some issues sending different emails with different recipients in my application, and while isolating tests I ran into the same issues. 

It seems to have to do with passing instance objects to `Mail.register_interceptor`.
*That* might not be the best approach.

See: https://github.com/mikel/mail/blob/master/lib/mail/mail.rb#L216-L220 
and: https://github.com/mikel/mail/blob/master/spec/mail/message_spec.rb#L1538-L1569