Tidy up the tests

* Use assert_nil where appropriate
* Lead with the expected value in collection_serializer_test.rb, etc
 so that expected/actual in test failure messages are not reversed

test/serializers/associations_test.rb

@@ -52,8 +52,8 @@ module ActiveModel
           serializer = association.serializer
           options = association.options
 
-          assert_equal key, :tags
-          assert_equal serializer, nil
+          assert_equal :tags, key
+          assert_nil serializer
           assert_equal [{ name: '#hashtagged' }].to_json, options[:virtual_value].to_json
         end
       end