Wrapped errors are not handled

[fabiante]

Hi there!

I'd like to use your middleware but noticed that it does not correctly handle wrapped errors. I opened a PR which adds a test case showing the behaviour #1.

I have not yet looked into why this happens. Before I tried your middleware, I used errors.Is to manually map custom errors onto http status codes. Generally the usage of errors.Is seems to be the right choice.

[richzw]

@fabiante Thank you very much for your issue. I think the reason is that only the first parameter of errors.is was Unwrap. Some comments were added to the PR.