unclobbering id builtin

1. removed refs to id builtin in favor of id_
2. removing trailing whitespace
3. using params.pop instead of params.get
- prevent id key from residing in params
- avoid overriding id builtin
- allow backwards compatibility; id, id_
4. adding APIResource compatibility integration test
5. Testing ListObject.retrieve with None values for id/id_

adding extra line (coding style)