Add a mock date utility for testing dates and timezone offsets.
Add support for parsing and emitting Date headers, with tests.
Add Date structured headers.
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.