Have Mail.deliver method uphold settings[:return_response]

[vkozyrev]

I have updated deliver to have consistent behavior with deliver!

This follows up on issue https://github.com/mikel/mail/issues/1225 My use case is I'm using sendgrid-actionmailer gem, and the response contains some data I'd like to have access too while still preserving the perform_deliveries setting.

Does this look like something you would be willing to merge in? Or are there any issues that I missed.

Currently 1 test failing after the update

rspec ./spec/mail/message_spec.rb:1826 # Mail::Message deliver should return self after delivery

I can update tests once I get some initial feedback