show: Unify JSON header output for messages and message parts

This has three ramifications:
- Blank To and Cc headers are no longer output for messages.
- Dates are now canonicalized for messages, which means they always
  have a day of the week and GMT is printed +0000 (never -0000)
- Invalid From message headers are handled slightly differently, since
  they get parsed by GMime now instead of notmuch.