net/http: Don't set Content-Length: -1 when responding to a POST

Fixes an issue where Response.Write writes out a Content-Length: -1
header when the corresponding Request is a POST or PUT and the
ContentLength was not previously set.

This was encountered when using httputil.DumpResponse
to write out the response from a server that responded to a PUT
request with no Content-Length header. The dumped output is
thus invalid.

Change-Id: I52c6ae8ef3443f1f9de92aeee9f9581dabb05991
Reviewed-on: https://go-review.googlesource.com/9496Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/responsewrite_test.go

@@ -207,6 +207,21 @@ func TestResponseWrite(t *testing.T) {
 			},
 			"HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n",
 		},
+
+		// When a response to a POST has Content-Length: -1, make sure we don't
+		// write the Content-Length as -1.
+		{
+			Response{
+				StatusCode:    StatusOK,
+				ProtoMajor:    1,
+				ProtoMinor:    1,
+				Request:       &Request{Method: "POST"},
+				Header:        Header{},
+				ContentLength: -1,
+				Body:          ioutil.NopCloser(strings.NewReader("abcdef")),
+			},
+			"HTTP/1.1 200 OK\r\nConnection: close\r\n\r\nabcdef",
+		},
 	}
 
 	for i := range respWriteTests {

src/net/http/transfer.go

@@ -138,6 +138,9 @@ func (t *transferWriter) shouldSendContentLength() bool {
 	if t.ContentLength > 0 {
 		return true
 	}
+	if t.ContentLength < 0 {
+		return false
+	}
 	// Many servers expect a Content-Length for these methods
 	if t.Method == "POST" || t.Method == "PUT" {
 		return true