Add mock date utility for testing dates and timezone offsets.

--HG--
extra : amend_source : 4c1537ec9b3932617553c3200f2becec409ed78a
Add support for parsing and emitting Date headers, with tests.

--HG--
extra : rebase_source : 1bdf06ba16d9aa4eee6a549060bfbf564d2b847b
Add Date structured headers.

--HG--
extra : rebase_source : 38172bead6d0fb5833d09afbc065ad0c24107dce