net/http: avoid copy if WithContext attempts to set exact same *ctx object

[kevinburke1]

Prior to NewRequestWithContext a lot of code looked like:

req, _ := http.NewRequest("GET", "http://example.com", nil)
req = req.WithContext(ctx)

Now that NewRequestWithContext is a thing, existing code may get ported - incorrectly - as:

req, _ := http.NewRequestWithContext(ctx, "GET", "http://example.com", nil)
req = req.WithContext(ctx)

Under the hood, WithContext does this:

func (r *Request) WithContext(ctx context.Context) *Request {
	if ctx == nil {
		panic("nil context")
	}
	r2 := new(Request)
	*r2 = *r
	r2.ctx = ctx
	r2.URL = cloneURL(r.URL) // legacy behavior; TODO: try to remove. Issue 23544
	return r2
}

Basically, my suggestion is to check if the incoming ctx and the existing r.ctx point to the same value in memory, then avoid the copy to r2 and just return r in that case.

In theory this is a breaking change of behavior if someone is doing e.g.

req2 := req.WithContext(ctx)

and then using both req and req2 though I don't think the behavior is very well defined if you do that.