Infers entity representer when just collection representer is specified.

When I use the ``represents`` method specifiying the collection representer only, RepresenterComputer fails to infer the entity representer.

The rules I followed:

- SingersController with ``represents :json, :entity => SingerRepresenter``: infers SingersRepresenters for the collection
- SingersController with ``represents :json, :collection => SingersRepresenter``: infer SingerRepresenter for the entity