Support passing of SelectManager to attribute methods

- Previously the passed `SelectManager` would get mistaken for an attribute and ultimately converted to a string. I added a test to exercise that but I'm not sure where's the best place to put it. For now it lives in the attribute tests (#gt) because that's how I discovered it.

- I removed `Arel::Nodes::SelectStatement` from the list because it's already covered by checking for `Arel::Nodes::Node`.

cc @tenderlove