create でやったことを踏襲すると update はこうなると思うのですが、これでは駄目なんでしょうか?
```
describe "PATCH /books/1" do
  context "valid params" do
    let(:book) { FactoryGirl.attributes_for(:book) }
    before { patch :update, id: book.id, book: book }
    it { response.should redirect_to(books_path) }
  end

  context "invalid params" do
    let(:book) { FactoryGirl.attributes_for(:book, isbn: nil) }
    before { patch :update, id: book.id, book: book }
    it { expect(response).to render_template(:edit) }
  end
end
```