HaveCodeError can panic

[didil]

I think this type assertion should use a safe type assertion

serr,ok := err.(*RequestError)

as it currently stands if I do a downstream error check using HaveCodeError and it's not a RequestError it panics.
I could do the .(*RequestError) check downstream but the general Go guideline is that library code shouldn't panic ...
Other than that thanks for this helpful library !

[pioz]

Thanks for reporting this issue. You are right! I've fixed the problem with this commit. Can you check on master if the problem is really fixed?

[didil]

great thanks !