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Fixed outdated stub that was sometimes breaking ci
deal with missing closing boundary
simplify encoding splitter and support whitespace inside encoded
universal splitting solution
Merge pull request #776 from zendesk/grosser/double-strip

simplify encoding splitter and support whitespace inside encoded
Merge pull request #757 from zendesk/grosser/missing-boundary

deal with missing closing boundary
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
extract converter
add BestEffortCharsetEncoder
fix more random errors
Fix spelling
Merge pull request #788 from gringocl/fix-spelling-in-readme

Fix spelling
Merge pull request #785 from zendesk/grosser/converter

Configurable charset encoders. Default to strict charset/encoding match. Provide a "best effort" encoder that may be lossy.
Merge pull request #787 from zendesk/grosser/rand

fix more random errors
avoid failed encodings / stop bad charsets early