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.
Fixed renderSortingLink not defaulting to property

Fixed renderSortingLink not defaulting to using the property name when sortable is set to false.
Merge pull request #2202 from LewisW/master

[ResourceBundle] Fixed renderSortingLink not defaulting to property name