Support compatibility when impl *http.Transport is used.

[drodriguezhdez]

In docker/distribution is being used the implementation *http.Transport instead of interfacehttp.RoundTripper, so it seems that it is not interchangeable

https://github.com/scope-demo/distribution/blob/master/notifications/http.go#L31:L46

[tonyredondo]

We can try to wrap their headerRoundTripper:

With our Transport instead of using the http.DefaultTransport: