`weak_dispatch` won't work for reference-retuning overloads

[mingxwa]

The follow code is well-formed but will be rejected:

#include <proxy/proxy.h>

PRO_DEF_MEM_DISPATCH(MemFun, Fun);

struct MyFacade : pro::facade_builder
    ::add_convention<pro::weak_dispatch<MemFun>, int&()>
    ::build {};

int main() {
  static_assert(pro::proxiable<int*, MyFacade>);
}