Make sure response is fully consumed

[mitar]

This fully consumes a response body. I have learned through some other experiences that this is important to do in Go. This is especially useful if nil is provided as v, because then response body is just closed. But also when JSON decoder does not decode all data (and there is some extra bytes after the decoded JSON response) - JSON decoder does not necessary consume full input, just enough that v is decoded. Any errors returned by io.Copy are ignored on purpose.

See similar thing in go-github library: google/go-github#317

[svanharmelen]

I'll take it, thanks 👍🏻