Thoughts on this for a nil validation spec:
    it "should cleanup nil attributes" do
      @simple_db_mock.should_receive(:select).
                      with("select * from stacks where itemName() = 'test-stack-us-west-1'").
                      and_return('test-stack-us-west-1' => { 'key1' => ['value1'] })
      @simple_db_mock.should_receive(:put_attributes).
                      with("stacks",
                           "test-stack-us-west-1",
                           { "Name"      => "test-stack-us-west-1",
                             "CreatedAt" => "2012-10-22 13:30:00 UTC" },
                           { :replace => ["Name", "CreatedAt"] } )
      @simple_db_mock.should_receive(:delete_items).with("stacks", "test-stack-us-west-1", {"key1"=>nil}
)
      @entry.set_attributes(['key1' => 'nil'])

      @entry.save
    end