zendframework/zf2 — pull request #3529 by Slamdunk (2013-01-23 08:06:38+00:00)
Title: Mail Sender: no header when empty

As of yet, Sender header is **always** writed in a mail message, becase it always returns a non-empty string, and so `Zend\Mail\Headers::toString()` can't skip it.

With this PR, just like `Zend\Mail\Header\AbstractAddressList`, now Sender will return empty string if no sender address is specified.