Support the null sender in the envelope from element class

Added null sender to from address machine
Spec to verify that sending with a sender of <> is allowed
Added a Mail::Constants::NULL_SENDER for convenience
Fixed outdated stub that was sometimes breaking ci
Merge pull request #745 from raindrift/master

Support for null envelope sender

Conflicts:
	lib/mail/constants.rb
	spec/mail/fields/resent_date_field_spec.rb