Support RFC1123 Date/Time in Retry-After header

[justenwalker]

According to RFC7231
the Retry-After header may be specified in both number of seconds OR an HTTPDate

The value of this field can be either an HTTP-date or a number of seconds to delay after the response is received.

   Retry-After = HTTP-date / delay-seconds

A delay-seconds value is a non-negative decimal integer, representing time in seconds.

   delay-seconds  = 1*DIGIT

Two examples of its use are

  Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
  Retry-After: 120

There are a couple PRs open to address this already.

• kmartin/Add support for RFC1123 Date/Time in Retry-After header. #130 by @kevinmartin (Was out of date at the time I wrote support HTTPDate in Retry-After header #138)
• support HTTPDate in Retry-After header #138 (Mine @justenwalker)
• Add support for RFC1123 Date/Time in Retry-After header. #143 by @Opelord - Another implementation inspired by kmartin/Add support for RFC1123 Date/Time in Retry-After header. #130

Clearly there is some interest in supporting this aspect of the RFC. So just to close this out I think we'd either like to see:

• One of the these PRs be accepted/merged
• Communication that this feature is deliberately absent and will not be accepted. In this case, perhaps we could add comment in the code about that decision to dissuade future contributors from creating additional PRs for this feature.