Exotic http lib leak not handled by goth

[dmoklaf]

The http lib has an exotic case of connection leak which is not taken care of by goth :
if for some reasons (e.g., ongoing production incident) the target authentication URL is redirecting too many times, the golang http client might return a non-nil response object together with a non-nil error.

This is an under-documented weakness of the Golang version 1 libraries, left for compatibility reasons, causing connection leak in many projects.

Consequently, when checking for an http client error, the goth code should also check if the response is non-nil, and if non-nil close its body.

[markbates]

Please submit a PR, I'd happily merge it.

[rakesh-eltropy]

@rgeronimi It is not fixed for all the providers. We still have same issue in dropbox, box, digital ocean, lastfm, linkedin, salesforce, spotify, twitch, twitter and yammer providers.

[dmoklaf]

Ah thanks, I hacked a simple shell script to search for such errors in my dependencies, and I missed the http Do pattern.
Fixed now, submitting a second PR.

[rakesh-eltropy]

@rgeronimi can you please close this.