Move find_target to more correct place for disable joins

Instead of redefining this in has_one_association we can define this in
singular association and reuse the existing `find_target` method.

Author: eileencodes

activerecord/lib/active_record/associations/has_one_through_association.rb

@@ -6,12 +6,6 @@ module Associations
     class HasOneThroughAssociation < HasOneAssociation #:nodoc:
       include ThroughAssociation
 
-      def find_target
-        return scope.first if disable_joins
-
-        super
-      end
-
       private
         def replace(record, save = true)
           create_through_record(record, save)

activerecord/lib/active_record/associations/singular_association.rb

@@ -38,7 +38,11 @@ def scope_for_create
         end
 
         def find_target
-          super.first
+          if disable_joins
+            scope.first
+          else
+            super.first
+          end
         end
 
         def replace(record)