Clean up handling of message headers.
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.

jordan-wright/email,ready4god2513/email,teepark/email,atsuyim/email,beni55/email