Honor configured :retry_delay over Retry-After

[Sim-hu]

Fixes #463.

The docs say :retry_delay should take precedence when it is explicitly set,
but 429/503 responses were still using Retry-After first.

This changes the retry delay lookup so that an explicit :retry_delay wins,
and Retry-After is only used when :retry_delay is not set.

Also adds a regression test for the case where both are present.