Merge resource specs into one spec file.

We had two types of resource spec files. One model spec and one library spec.
Since the resource class is a lib and no model, the specs should be at one place.
Also this removes duplicated specs defined in both spec files.
Fixes bugs in resource_column_type implementation.

1. Not all db adapters respond_to :array
2. If the column is a relation, the type was always return as :array, instead of the column's type
Renames resource's `skip_attributes` into `skipped_alchemy_resource_attributes`.

This prevents name collisions in model class.
Use Rails' default exception handling while in page preview.

When we are in page preview while editing a page, we should not rescue the exceptions.
We should use the default Rails behaviour of showing a meaningful error explanation in development mode and the 500 page while in production mode.
Fixes for RSS feeds.