It should be invalid to go :through a polymorphic belongs_to

Consider the following:

    class Tagging
      belongs_to :taggable, :polymorphic => true
      has_many :something, :through => :taggable
    end

Obviously the something association on the taggable might only be available at certain times, depending on what the taggable actually is.

At the moment, the above example fails, because it tries to look up the source reflection, which involves getting the through_reflection.klass, which doesn't exist.

Whilst it would be theoretically possible to support the above, it's basically insane. I suggest we add in a HasManyCantAssociateThroughPolymorphicBelongsToReflection and raise it properly in ThroughReflection#check_validity!.