Add format for imf-fixdate used in HTTP headers

[mikekistler]

This PR adds a new format for the date-time format used in HTTP headers, as described in RFC 7231, Section 7.1.1.1.

[MikeRalphson]

LGTM, thanks! As I wasn't familiar with this RFC, I thought maybe a rendered example of a date-time in this format might be beneficial, but I realise we don't have them for most of the other formats (that's my bad, it's missing on the original template).

[mikekistler]

I like the suggestion of adding an example so I have done that. I also clarified that this is the preferred format for dates in headers, to match the wording in the RFC.