http: remove finalURL from Client.Get; move to Response

http: remove finalURL from Client.Get; move to Response

This CL:

-- removes Response.RequestMethod string
-- adds Response.Request *Request
-- removes the finalURL result parameter from client.Get()
-- adds a gofix rule for callers of http.Get which assign
   the final url to the blank identifier; warning otherwise

Caller who did:

res, finalURL, err := http.Get(...)

now need to do:

res, err := http.Get(...)
if err != nil {
   ...
}
finalURL := res.Request.URL.String()

R=rsc
CC=golang-dev
https://golang.org/cl/4535056