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

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extra : rebase_source : 0e531171a09e5b20d383929fad8dd116765b7a23
extra : amend_source : 7b132e7c62d3443eaf97f009c53a9b7e9a7c0001
Add Date structured headers.

--HG--
extra : rebase_source : 314094d70b127a47428713d68316732cd9f3ffb3