mail: convert encoding.encoding to sysstr

mail: call s.decode('ascii') explicitly to see if s is an ascii bytes

mail: pass in addr to _addressencode() in bytes _addressencode() doesn't process a unicode addr well. For example, acc.encode('ascii') would raise UnicodeEncodeError if acc was a unicode. This patch temporarily restores the pre-952bf3c948f0 _addressencode(). Several tests would be broken on Python 3.