Simplify http date parser according to RFC 7231

[ansiwen]

Before the date has been parsed according to RFC 1123. However, as
pointed out by issue #91, RFC 7231 defines a subset of this format
for HTTP dates (IMF-fixdate). This change simplifies the parser,
assuming the IMF-fixdate format according to RFC 7231. In violence
to RFC 7231 the legacy RFC 850 and asctime() formats are
intentionally not handled, as before.

Closes #91

[ansiwen]

@seliopou ping

[seliopou]

IIRC this is restricting the possible allowed inputs for the parser? That kindof runs counter to the Robustness Principle. Unless I'm missing something, I don't think this is a change worth making.

[hannesm]

@seliopou in my opinion, Postel's law is wrong ;). in any case, I struggle with Scanf quite a bit, and would instead appreciate to have e.g. a dependency on Angstrom und use the parser combinators from there. My experience with scanf is that it accepts arbitrary trailing input. Similar to #93 I guess we should add test cases from the RFC to convince ourselves that the parser is not too strict.

[seliopou]

Closing for now, we can revisit the change if/when the parser gets rewritten using angstrom.