Correctly handle `Retry-After` header with `503 Service Unavailable`

[austind]

Currently, retryhttp makes the false assumption that any response that contains a Retry-After header must have been rate limited. In fact, Retry-After indicates planned downtime when accompanied by 530 Service Unavailable.

See MDN docs on Retry-After for details.

[RichieB2B]

I think you made a typo and meant to say 503 Service Unavailable in the description.