projecting join: fix schema with columns are projected

If we keep columns from the right child, but project them away, we may
not need to rename them to avoid duplicates.

@7andrew7 any thoughts about this change?
This makes sense.  The code is subtle, as it relies on the name mangling logic of the Scheme class.  My preference would be to move the name mangling logic out of Scheme.  In practice, most of the name resolution logic is in algebra.py, so moving all naming logic to algebra.py makes the code clearer.

Obviously, consider this an optional enhancement.  We should add a test case, however.