Updated to describe defaulting to property name behaviour

Updated the spec for the Twig extension, so it describes the behaviour that happens when rendering a sorting link with no field name.
I've also renamed the "it_should_not_render_sorting_link" to "it_should_not_render_sorting_link_when_not_sortable" so it's more consistent with the new scenario name, and also a little more explicit about what it does.