Added JSON_NUMERIC_CHECK as default on toJson

This was previously the default but appears to have gone missing in a previous commit.

The problem with not having this as the default is it's impossible to pass JSON options to an Eloquent object if its being returned with out an explicit `toJson` call. Maybe as an alternative there should be a `defaultJsonOptions` attribute on the `Eloquent\Model` class which can be overridden in the models where this is needed?