Ignore ErrNotFound while matching Subrouters

[g-w]

MatchErr is set by the router to ErrNotFound if no route matches. If
no route of a Subrouter matches the error can by safely ignored. This
implementation only ignores these errors and does not ignore other
errors like ErrMethodMismatch.

[kisielk]

What if you have a router whose only routes are subrouters, and all of them return ErrNotFound?

[g-w]

In this case no route should match and https://github.com/gorilla/mux/blob/master/mux.go#L167 should be reached. This sets MatchErr to ErrNotFound.

I've checked this through this test:

func TestMethodSubrouterNotFound(t *testing.T) {
	handler := func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusOK) }
	router := NewRouter()
	router.Path("/a").Subrouter().HandleFunc("/thing", handler).Methods(http.MethodGet)
	router.Path("/b").Subrouter().HandleFunc("/something", handler).Methods(http.MethodGet)

	w := NewRecorder()
	req := newRequest(http.MethodPut, "/not-present")

	router.ServeHTTP(w, req)

	if w.Code != 404 {
		t.Fatalf("Expected status code 404 (got %d)", w.Code)
	}
}

If you think it useful, I will add it to my PR.

BTW: This fixes #437

[kisielk]

Yes I think adding this test is useful just to verify that this and any future changes won't break this.

[g-w]

I've added the test.

[elithrar]

LGTM

[elithrar]

Thanks @g-w!