[FIX] Extension of controller in-place with explicit spec of same _cp_path

[FIX] Extension of controller in-place with explicit spec of same _cp_path

When extending a controller in-place (e.g. A(Controller), B(A)) and
providing the exact same _cp_path as parent (no-op) execution path
would go into handler for _cp_path overwriting and raise an assertion
error for overwriting of existing controller.

Except this is allowed (if ugly) pattern, so warn & ignore behavior
(it is harmless).

bzr revid: xmo@openerp.com-20130418092405-wrmmrd648b9koefu