Fixed a bug when Retry-After is 0, it should use min wait time.

[evanchaoli]

In my test, I set rate limit block_interval=5 rate=1, then the first response had Retry-After: 4 and the second response had Retry-After: 0, so that the second retry still failed.

When server responds Retry-After: 0, tests prove that immediate retry will fail also. Thus I think in that case MinWaitTime should be used.

The other approach I was thinking is to always return Retry-After + 1. When rate is 1, that should work better. As I my test case rate limit set to block_interval=5 rate=1, when hit rate limit, first 429 response will have Retry-After: 4, after 4 seconds, retry will hit 429 again with Retry-After: 0, then if retry immediately, if will hit 429 again. Thus if backoff is Retry-After + 1, that should work better.

But if block_interval=1, if we do Retry-After + 1, then retry interval will always be 1 second, which sounds not good, thus I chose the current approach I implemented in this PR.

[hashicorp-cla]

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