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

src/pkg/http/fs_test.go

@@ -96,7 +96,7 @@ func TestServeFileContentType(t *testing.T) {
 	}))
 	defer ts.Close()
 	get := func(want string) {
-		resp, _, err := Get(ts.URL)
+		resp, err := Get(ts.URL)
 		if err != nil {
 			t.Fatal(err)
 		}