avoid dynamic finders on an assocition with an include

the dynamic finder on the association causes scoping to be called
before delegating the dynamic finder to the class, which works
until the preloader tries to load the owning class, and BOOM,
tries to preload itself

fixes at least one spec for rails 3

Change-Id: Ia22b31a013cb6ac984eb77ca29406372538ea57f
Reviewed-on: https://gerrit.instructure.com/30393
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>