Add support for parsing and emitting Date headers, with tests.

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extra : rebase_source : 1bdf06ba16d9aa4eee6a549060bfbf564d2b847b
extra : amend_source : 906006fb8847a51e0868dfee2e748d094972cca8
Add Date structured headers.

--HG--
extra : rebase_source : ca7056d21e4675f6a132253a37c4c440a1cab34e