net/http: accept empty method in Transport again

Fix regression from https://golang.org/cl/16829 ("require valid methods
in NewRequest and Transport.RoundTrip").

An empty string is a valid method (it means "GET", per the docs).

Fixes #13311

Change-Id: I26b71dc4ccc146498b5d7e38fbe31ed11dd5a6cf
Reviewed-on: https://go-review.googlesource.com/16952
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/http/transport.go

@@ -237,7 +237,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		req.closeBody()
 		return nil, &badStringError{"unsupported protocol scheme", s}
 	}
-	if !validMethod(req.Method) {
+	if req.Method != "" && !validMethod(req.Method) {
 		return nil, fmt.Errorf("net/http: invalid method %q", req.Method)
 	}
 	if req.URL.Host == "" {

src/net/http/transport_test.go

@@ -20,6 +20,7 @@ import (
 	"net"
 	. "net/http"
 	"net/http/httptest"
+	"net/http/httputil"
 	"net/url"
 	"os"
 	"reflect"
@@ -1775,7 +1776,6 @@ func TestTransportNoHost(t *testing.T) {
 	defer afterTest(t)
 	tr := &Transport{}
 	_, err := tr.RoundTrip(&Request{
-		Method: "GET",
 		Header: make(Header),
 		URL: &url.URL{
 			Scheme: "http",
@@ -1787,6 +1787,19 @@ func TestTransportNoHost(t *testing.T) {
 	}
 }
 
+// Issue 13311
+func TestTransportEmptyMethod(t *testing.T) {
+	req, _ := NewRequest("GET", "http://foo.com/", nil)
+	req.Method = ""                                 // docs say "For client requests an empty string means GET"
+	got, err := httputil.DumpRequestOut(req, false) // DumpRequestOut uses Transport
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(got), "GET ") {
+		t.Fatalf("expected substring 'GET '; got: %s", got)
+	}
+}
+
 func TestTransportSocketLateBinding(t *testing.T) {
 	defer afterTest(t)