Fix head break connection

[mehanizm]

Problem

I tried to use grab and tested download from server https://speed.hetzner.de
When I retried download to partially saved file grab failed with nil response and error Head EOF

I've researched the source of the problem. Remote server breaks the connection on HEAD request, but works good at GET request. So, I think that in this case grab should not fail after HEAD and go further and try GET.

This PR fixes this.

Solution

Test

I've added test that emulate HEAD request breaking connection WithHeadRequestBreak.

Fix

In headRequest State Function in branch with response error grab now does not closing the response but go to GET request.

client.Do(req) returns only nil response along with error, so there is no need to close nil response.

Other fixes

I've added one more commit to PR with linter fixes and update of the Go version.