Email::Sender::Transport::SMTP::Persistent should call its disconnect method on destruction

[abraxxa]

When using the SMTP::Persistent transport, the last mail of a connection is dropped by the mail relay connected to, when the disconnect method isn't called.

As the disconnect method only exists for the SMTP::Persistent transport, code that allows configuring the transport would have to check if the transport instance has a disconnect method, and call it if.
As that isn't nice I'd prefer if it gets called automatically when the transport instance is destroyed, end of program in my case.

If that's not safely possible on global destruction, maybe moving the reset from start of next mail to end of mail would fix the problem of dropped last mail.