Module#constants

Specs fixed:

Module#constants
- returns all constants including inherited when passed true
- returns all constants including inherited when passed some object
- doesn't returns inherited constants when passed false
- doesn't returns inherited constants when passed nil
Module.constants

Specs fixed:

Module.constants
- returns an array of the names of all toplevel constants
- returns an array of Symbol names
make RPython happy :)
Merge pull request #697 from krekoten/module_constants

Module#constants and Module.constants