Add Net::SMTP::Address#to_str

[bquorning]

Add implicit conversion from Net::SMTP::Address to String. Test both explicit and implicit conversion from Net::SMTP::Address to String.

The implicit conversion can be useful when using Mail::SmtpEnvelope with a Net::SMTP::Address instance instead of a String, and it’s being passed to Regexp#match? in https://github.com/mikel/mail/blob/73db11a6463ec330a36d68cfb77ef99511126cb0/lib/mail/smtp_envelope.rb#L50