net/http: be more consistent about Request.Method "" vs "GET"

Patch from Russ.

No bug identified, but I didn't search exhaustively. The new code is
easier to read.

Fixes #13621

Change-Id: Ifda936e4101116fa254ead950b5fe06adb14e977
Reviewed-on: https://go-review.googlesource.com/17981Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/client.go

@@ -179,10 +179,11 @@ func (c *Client) send(req *Request) (*Response, error) {
 //
 // Generally Get, Post, or PostForm will be used instead of Do.
 func (c *Client) Do(req *Request) (resp *Response, err error) {
-	if req.Method == "" || req.Method == "GET" || req.Method == "HEAD" {
+	method := valueOrDefault(req.Method, "GET")
+	if method == "" || method == "GET" || method == "HEAD" {
 		return c.doFollowingRedirects(req, shouldRedirectGet)
 	}
-	if req.Method == "POST" || req.Method == "PUT" {
+	if method == "POST" || method == "PUT" {
 		return c.doFollowingRedirects(req, shouldRedirectPost)
 	}
 	return c.send(req)

src/net/http/request.go

@@ -1057,11 +1057,13 @@ func (r *Request) closeBody() {
 }
 
 func (r *Request) isReplayable() bool {
-	return r.Body == nil &&
-		(r.Method == "GET" ||
-			r.Method == "HEAD" ||
-			r.Method == "OPTIONS" ||
-			r.Method == "TRACE")
+	if r.Body == nil {
+		switch valueOrDefault(r.Method, "GET") {
+		case "GET", "HEAD", "OPTIONS", "TRACE":
+			return true
+		}
+	}
+	return false
 }
 
 func validHostHeader(h string) bool {

src/net/http/transfer.go

@@ -56,7 +56,7 @@ func newTransferWriter(r interface{}) (t *transferWriter, err error) {
 		if rr.ContentLength != 0 && rr.Body == nil {
 			return nil, fmt.Errorf("http: Request.ContentLength=%d with nil Body", rr.ContentLength)
 		}
-		t.Method = rr.Method
+		t.Method = valueOrDefault(rr.Method, "GET")
 		t.Body = rr.Body
 		t.BodyCloser = rr.Body
 		t.ContentLength = rr.ContentLength