Additional BooksController.index() tests

[gaurijo]

This PR addresses issue 33:

In BooksController.index(), there are four formats in the respond_to block: HTML, CSV, JSON, and XML. We should have tests for each of these that check for an HTTP 200 response, check the Content-Type header, and ideally check that the response body looks like valid HTML/CSV/JSON/XML.

Each format (HTML, CSV, JSON, and XML) tests for the following:

• a 200/successful http response
• expected Content-Type header
• valid response body