The MockDate .getTimezoneOffset() was reversed from ES 6 sense.
Replace the generator with a returned list, for speed.

It appears that SpiderMonkey currently doesn't JIT optimize generators, so
converting it to lists appears to achieve somewhere around 10-20% speedups.
Handle non-addrspec addressing headers better.
Prevent some JS strict warning errors.
Fix minor issues to get Thunderbird's version of these tests passing.
Pull the quoting code out into a helper method.
Add support for emitting parameter-based headers.
Add support for emitting Content-Type header as a structured header.
Add support for Content-Disposition headers.