checking for headers before trying to set on them
Merge pull request #6 from barnjamin/master

checking for headers before trying to set on them
Add big ugly test.

Verifies compliance of existing expected behavior, but will change soon.
Make message bodies []byte instead of string
Clean up handling of message headers.

- msgHeaders() curates fields from an Email along with its Headers map
  to create a sane set of headers for the email.
- headerToBytes() creates duplicate fields now rather than combining
  multiple values into one field, per RFC 2822 sec. 3.6.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	email.go