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>