Doer: Inline the Doer type at least for now

transport.go

@@ -104,18 +104,14 @@ func (cl closeLogger) Close() error {
 	return cl.ReadCloser.Close()
 }
 
-// Doer is an interface with a Do method.
+// DoerTransport converts a Doer into a Transport.
 // It exists for compatibility with other libraries.
+// A Doer is an interface with a Do method.
 // Users should prefer Transport,
 // because Do is the interface of http.Client
 // which has higher level concerns.
-type Doer interface {
+func DoerTransport(cl interface {
 	Do(req *http.Request) (*http.Response, error)
-}
-
-// DoerTransport converts a Doer into a Transport.
-func DoerTransport(cl Doer) Transport {
-	return RoundTripFunc(func(req *http.Request) (*http.Response, error) {
-		return cl.Do(req)
-	})
+}) Transport {
+	return RoundTripFunc(cl.Do)
 }