Concurrency issue: call .Do() in different goroutines replace CheckRedirect of singleton client

[vvelikodny]

I found a bug when call .Do() func in some goroutines concurrently which process requests with redirects.

Each time we call .Do(), CheckRedirect will be replaced here

So any .Do() call with request which have redirect can replace redirectFailed value of other request without redirect, so .Do() return results which change each other.

[marcosnils]

@vvelikodny good catch, would you like to submit a PR to fix it?