add possibility to unhide all attributes on an Eloquent model

Rationale: I have a CSV export feature in my application which is for administrators only. They need certain data to be in the export which at no time needs to be visible in the REST API I have built. So here I have added the possibility to unhide/rehide the values in the $hidden array.

Problems : unhide is not an actual word. But it gets the point across, and it does convey the idea of being opposed to the default action of hiding stuff in the $hidden array.

PR against 4.1 as it is non-breaking.