Avoid lambda scopes when possible

Lambda scopes require a bunch more work.  Ideally the `scope` list would
be a homogeneous collection.  In this case, the reflection knows how to
construct the right reflection for this join, so lets just construct
that relation rather than instance execing to figure it out later.

activerecord/lib/active_record/reflection.rb

@@ -807,9 +807,7 @@ def scopes
       end
 
       def source_type_scope
-        type = foreign_type
-        source_type = options[:source_type]
-        lambda { |object| where(type => source_type) }
+        through_reflection.klass.where(foreign_type => options[:source_type])
       end
 
       def has_scope?