isbnの時はたまたまamazon_api_mock.rbの
```ruby
shared_context "amazon_api_mock" do
  before do
    Amazon.any_instance.stub(:book_title) { "列車本" }
    Amazon.any_instance.stub(:book_image_url) { 'http://ecx.images-amazon.com/images/I/51Gibg-iYTL._SL75_.jpg' }
  end
end
```
のany_instance.stubに置き換わらなかったのでたまたま正直な方法でうまく行っていましたが、
:title,:image_urlの時はstubの:book_title,:book_image_urlに置き変わってしまうため、
```ruby
context "invalid params" do
  let(:edited_book) { FactoryGirl.attributes_for(:book, title: nil) }
  before { patch :update, id: book.id, book: edited_book }
  it { expect(response).to render_template(:edit) }
end
```
とテストを記述しても、books_controller内では
```ruby
23: def update
    24:   @book = Book.find(params[:id])
 => 25:   binding.pry
    26:   if @book.update_columns(update_book_params)
    27:     redirect_to books_path, notice: 'update book'
    28:   else
    29:     render :edit
    30:   end
    31: end

[1] pry(#<BooksController>)> @book
=> #<Book id: 1, title: "列車本", isbn: "4774157198", created_at: "2013-06-19 08:57:46", updated_at: "2013-06-19 08:57:46", image_url: "http://ecx.images-amazon.com/images/I/51Gibg-iYTL._...">
```
となってしまってテストがうまく記述できません。