Don't change the slug during validation

As the slug for an artefact was being passed as a reference into
the slug validator, slugs for detailed guides were being changed
during validation when we replaced any double dashes with a single
dash.

This makes sure that, when we pass the slug value directly into the
validator, we clone it so that any changes don't affect the value
in the Artefact.
Merge pull request #110 from alphagov/stop-clobbering-inside-gov-slugs

Don't change the slug during validation