Unsecure tls fix

@ryansb after merging your last change I kept thinking if there's really a reason why we should provide goreq users to send transport through Request struct as what we want to achieve is simplify request handling. 

I've noticed that the only place where we were using it was in the TLS test as we needed to assert that TLS verification has been enabled. I've managed to write the test in a different way which doesn't require to initialize goreq with a transport. 

What do you think about this approach?