belongs_to association fixes

This fixes two bugs:

* For polymorphic belongs_to associations, the `replicate_reflection_info` method was using `Kernel.const_get` to look up the reference class. This failed on namespaced class names.
* For omitted belongs_to associations, the association information was still looked up rather than simply leaving the foreign key as-is. For example, an omitted `belongs_to :user` would write `[:id, "User", 1]` instead of just `1`.