Freeze string
Fix #label specs with has_many associations and

improve #label test coverage.

Fixed specs: searches/labels for `has_many` associations
need to use pluralized model_name (was singularized).

Test for finding the translation when provided in the I18n file,
as well for correctly falling back (with coherent capitalization)
in different cases when no translation is provided.

Test with direct attributes (both one word and multi-word),
as well as with `has_many` and `belongs_to` associations.

To-do: improve Ransack I18n documentation.