oauth2: use request.Clone() instead of a custom clone implementation

Also fixes #457

transport.go

@@ -47,7 +47,7 @@ func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
 		return nil, err
 	}
 
-	req2 := cloneRequest(req) // per RoundTripper contract
+	req2 := req.Clone(req.Context()) // per RoundTripper contract
 	token.SetAuthHeader(req2)
 
 	// req.Body is assumed to be closed by the base RoundTripper.
@@ -73,17 +73,3 @@ func (t *Transport) base() http.RoundTripper {
 	}
 	return http.DefaultTransport
 }
-
-// cloneRequest returns a clone of the provided *http.Request.
-// The clone is a shallow copy of the struct and its Header map.
-func cloneRequest(r *http.Request) *http.Request {
-	// shallow copy of the struct
-	r2 := new(http.Request)
-	*r2 = *r
-	// deep copy of the Header
-	r2.Header = make(http.Header, len(r.Header))
-	for k, s := range r.Header {
-		r2.Header[k] = append([]string(nil), s...)
-	}
-	return r2
-}