net/http: minor optimization to copyValues

[rsc]

net/http has:

func copyValues(dst, src url.Values) {
	for k, vs := range src {
		for _, value := range vs {
			dst.Add(k, value)
		}
	}
}

This should probably be:

func copyValues(dst, src url.Values) {
	for k, vs := range src {
		dst[k] = append(dst[k], vs...)
	}
}

Noted on #33498

[gopherbot]

Change https://golang.org/cl/191057 mentions this issue: net/http: minor optimization to copyValues

[gopherbot]

Change https://golang.org/cl/191278 mentions this issue: Master