type Transport struct {
	// Proxy takes an http.Request and provides a URL to use for that request.
	// Note that the semantics are different from http.DefaultTransport: this
	// proxy is always invoked. If Proxy is nil, requests to the Transport are
	// unaltered.
	Proxy func(*http.Request) (*url.URL, error)

	// Next is the http.RoundTripper to which requests are forwarded.  If Next
	// is nil, http.DefaultTransport is used.
	Next http.RoundTripper
}