portage/mail.py: don't encode Date header to bytes

As reported in bug #520752 comment #4, the _unicode_encode usage
triggers the following error with python 3.3.5:

	'bytes' object has no attribute 'encode'"

The _unicode_encode call is not needed for python2.x, since formatdate
returns bytes for python2.x.

Fixes: 7204b87040ed ("portage/mail.py: Make email date, timestamp RFC-compliant Bug 520752")
X-Gentoo-Bug: 520752
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=520752