fix default_power_ids for nil powers

In my project I defined a power like

```
  power :clients do
    Client.active unless guest?
  end
```

and get this error
```
undefined method `collect_ids' for nil:NilClass (ActionView::Template::Error)
      .../ruby-1.8.7-p374/gems/consul-0.12.0/lib/consul/power.rb:47:in `default_power_ids'
```

This would fix it:
```
    def default_power_ids(power_name, *args)
      scope = send(power_name, *args)
      database_touched
      if scope
        scope.collect_ids
      else
        []
      end
    end
```

But I don't know yet, why the following tests don't break
```
  context 'nil powers' do

    describe 'query methods' do

      context 'when no record is given' do

        it 'should return false' do
          @user.role = 'guest'
          @user.power.clients.should be_nil
          @user.power.clients?.should == false
        end

      end

      context 'with a given record' do

        it 'should return false' do
          client = Client.create!
          @user.role = 'guest'
          @user.power.clients.should be_nil
          @user.power.client?(client).should == false
        end

      end

    end
```

and I don't want to release this patch without a reliable test!