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

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extra : rebase_source : 0e531171a09e5b20d383929fad8dd116765b7a23
extra : amend_source : 1dee407ef192145cd371d5d9ad4ae0cd6ac4ba75
Add Date structured headers.

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extra : rebase_source : c92ee926333d15e2ad41a39430b71caa8c486b71