Provide a default id field for all SObject subclasses #27

Fixes #27

Adds a default field called `id` (mapped to `Id`) to each subclass of `SObject`.

Change to association_spec was needed because the return value of `query` on a Restforce client returns a `Restforce::Collection` which is an Enumerable. This allows `SObject.build` to behave as expected when that test is run.