build association instance using _type if present
use _type for class when building new association
delete parent's (belongs_to) cached instances when saving/updating/deleting a model
tests that reflect the problem
belongs_to_keys should return parent's value when called from an inherited class
consider case where belongs_to uses custom class name
add new callbacks on new has_many association, use it to clear cache
Merge pull request #14 from lperichon/master

Add cache clearing on associations