Only remember that an association was loaded if it was found

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@435 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/lib/active_record/associations/association_proxy.rb

@@ -58,7 +58,7 @@ def load_target
               reset
             end
           end
-          @loaded = true
+          @loaded = true if @target
           @target
         end