Add support for Arrayable object columns

[27pchrisl]

This PR enables support for object columns that support Laravel's Arrayable interface.

When using https://github.com/spatie/laravel-data objects/collections as eloquent casts, there was an incompatibility with IsJsonRelation that expects the column to be an array type. This change simply calls toArray if the collection can convert to an array.

[staudenmeir]

Thanks, I'll take a look.

[27pchrisl]

This might fix #84 if it's the same sort of setup.

[staudenmeir]

Thanks, I released a new version.