net/http/httptest: match net/http ContentLength behavior for http.NoBody

Fixes #68476

Change-Id: I05122e5ec5e6b290eec93f3db444fcf1de19c030
Reviewed-on: https://go-review.googlesource.com/c/go/+/599815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Daniel Martí <mvdan@mvdan.cc>

src/net/http/httptest/httptest.go

@@ -34,9 +34,9 @@ func NewRequest(method, target string, body io.Reader) *http.Request {
 //
 // An empty method means "GET".
 //
-// The provided body may be nil. If the body is of type *bytes.Reader,
-// *strings.Reader, or *bytes.Buffer, the Request.ContentLength is
-// set.
+// The provided body may be nil. If the body is of type [bytes.Reader],
+// [strings.Reader], [bytes.Buffer], or the value [http.NoBody],
+// the Request.ContentLength is set.
 //
 // NewRequest panics on error for ease of use in testing, where a
 // panic is acceptable.
@@ -69,6 +69,9 @@ func NewRequestWithContext(ctx context.Context, method, target string, body io.R
 		default:
 			req.ContentLength = -1
 		}
+		if body == http.NoBody {
+			req.ContentLength = 0
+		}
 		if rc, ok := body.(io.ReadCloser); ok {
 			req.Body = rc
 		} else {

src/net/http/httptest/httptest_test.go

@@ -156,6 +156,24 @@ func TestNewRequestWithContext(t *testing.T) {
 			wantBody: "foo",
 		},
 
+		{
+			name:   "Post with NoBody",
+			method: "POST",
+			uri:    "/",
+			body:   http.NoBody,
+			want: &http.Request{
+				Method:     "POST",
+				Host:       "example.com",
+				URL:        &url.URL{Path: "/"},
+				Header:     http.Header{},
+				Proto:      "HTTP/1.1",
+				ProtoMajor: 1,
+				ProtoMinor: 1,
+				RemoteAddr: "192.0.2.1:1234",
+				RequestURI: "/",
+			},
+		},
+
 		{
 			name:   "OPTIONS *",
 			method: "OPTIONS",