[Enhancement] Customize date format of `RetryAfter` header

[boddie2]

Detailed description

I came across this date format 2020-11-10T18:44:00.000+00:00 in theRetryAfter header.

Context

It would be useful to have a possibility to specify the date format of this header.

Possible implementation

Add an option in the options array that allows us to specify the date format of this header. Default constant DATE_FORMAT

[Casey McLaughlin]

Hi @boddie2, I implemented this enhancement in v2.6; refer to the README file for documentation.