net/http: remove "number:" from Response.Status string

The behavior of Value.String method on non-string JavaScript types has
changed after CL 169757.

Update the implementation of Transport.RoundTrip method to construct the
Response.Status string without relying on result.Get("status").String(),
since that now returns strings like "<number: 200>" instead of "200".

Fixes #31736

Change-Id: I27b3e6cc95aa65fd1918b1400e88478a154aad12
Reviewed-on: https://go-review.googlesource.com/c/go/+/174218Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>

src/net/http/roundtrip_js.go

@@ -138,10 +138,11 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 			body = &arrayReader{arrayPromise: result.Call("arrayBuffer")}
 		}
 
+		code := result.Get("status").Int()
 		select {
 		case respCh <- &Response{
-			Status:        result.Get("status").String() + " " + StatusText(result.Get("status").Int()),
-			StatusCode:    result.Get("status").Int(),
+			Status:        fmt.Sprintf("%d %s", code, StatusText(code)),
+			StatusCode:    code,
 			Header:        header,
 			ContentLength: contentLength,
 			Body:          body,