Fix nil pointer dereference panic on TCP reqs:

Calling `Close()` closes the tpc socket, but the underlying lib is still expecting it to be open so it can [do some retry logic](https://github.com/miekg/dns/blob/5c48f3623a4b2f1465b8ccf4fe29b6eb3888ee07/server.go#L332-L356)

We either need to call `response.Hijack()` early – to let the underlying lib know that we're planning on handling `Close()` ourselves – or we need to just let it do its thing.

I've opted to leave it up to the lib completely, so it can do it's retry stuff if that's ever useful.
Merge pull request #15 from duedil-ltd/fix/tcp-panic

Fix nil pointer dereference panic on TCP reqs