Move retry logic into middleware

[whickman]

What happened?

Currently retry logic happens in the client at the layer above the middleware. Because error decoding a response body to be nil (see [0]), the retry logic cannot inspect headers and therefore cannot do any advanced handling of http codes >= 400.

What did you want to happen?

If the retry logic were to be moved into the middleware and run before the error decoding middleware, we would be able to inspect these headers.

[0] #78 (comment)