Limit lookups to specific classes via :only

Seems straightforward.  The ticket mentioned limiting via `klass < mod` but I would think `klass <= mod` an implemented accordingly...
Some suggestions :smile:

```ruby
def find(gid, options = {})
  if global_id = parse(gid)
    global_id.find if find_allowed_for?(global_id.model_class, options[:only])
  end
end

private
  def self.find_allowed_for?(klass, only = nil)
    if only
      Array(only).any? { |c| klass <= c }
    else
      true
    end
  end
```