Implement Action#format

Closes #15
All these tests are exercising the same code: the direct assigment of format.
They are good to test the symbol to mime type lookup.

However I'd add integration tests without the assigment. We set the `Accept` header to `application/json` and see if `#format` is able to return `:json` and `#content_type` `application/json`.

I see the usage of `#format=` not the common case, developers will leverage on the automatism described above.

What do you think?