Convert attribute type to symbol in searchable_attributes

While the usage description of the `alchemy_resource_relations` class method indicated to use strings as values, the `searchable_attributes` method was checking these values against a symbol. This check could never return true.
Fixes #694 eager load associations in query_items

The associations of a resource needs to be eager loaded when selecting a record and the columns of the associated models are involved.

If no association exists nothing will be eager loaded.