Always pass `inherit` param to `Module#constants`

This fixes the `ls Module` case, because it makes the singleton method
`Module.constants` call the instance method. :notes:
Add regression test for 28afdd547d712b5ea1292ec724f877706669c80f
Better fix for 28afdd547d712b5ea1292ec724f877706669c80f

That solution doesn't work on MRI 1.8.