Correctly parse empty Subject header

Without this fix parsing an email with an empty subject header "Subject:" fails.

Fatal error: Uncaught exception 'Zend\Mail\Header\Exception\InvalidArgumentException' with message 'Invalid header line for Subject string' in library/Zend/Mail/Header/Subject.php:39
[#3571] wrote test

- Created test that demonstrates empty subject header is correctly
  parsed
Merge branch 'hotfix/3571'

Close #3571