net/http/httputil: make https DumpRequestOut less racy

It's still racy in that it mutates req.Body, though.  *shrug*

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5709054

src/pkg/net/http/httputil/dump.go

@@ -12,6 +12,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 )
@@ -63,9 +64,13 @@ func DumpRequestOut(req *http.Request, body bool) ([]byte, error) {
 	// switch to http so the Transport doesn't try to do an SSL
 	// negotiation with our dumpConn and its bytes.Buffer & pipe.
 	// The wire format for https and http are the same, anyway.
+	reqSend := req
 	if req.URL.Scheme == "https" {
-		defer func() { req.URL.Scheme = "https" }()
-		req.URL.Scheme = "http"
+		reqSend = new(http.Request)
+		*reqSend = *req
+		reqSend.URL = new(url.URL)
+		*reqSend.URL = *req.URL
+		reqSend.URL.Scheme = "http"
 	}
 
 	// Use the actual Transport code to record what we would send
@@ -88,7 +93,7 @@ func DumpRequestOut(req *http.Request, body bool) ([]byte, error) {
 		},
 	}
 
-	_, err := t.RoundTrip(req)
+	_, err := t.RoundTrip(reqSend)
 
 	req.Body = save
 	if err != nil {