Use model that doesn't fail with race condition

For some reason, the post would sometimes be serialized as

  "{\"id\":\"1\",
+ \"type\":\"posts\", \"attributes\":{\"title\":\"New Post\",\"body\":\"Body\"},
  \"comments\":[{\"id\":1,\"body\":\"ZOMG A COMMENT\"}],
  \"blog\":{\"id\":999,\"name\":\"Custom blog\"},
  \"author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"

instead of:

  "{\"id\":1,
-  \"title\":\"New Post\",\"body\":\"Body\",
   \"comments\":[{\"id\":1,\"body\":\"ZOMG A COMMENT\"}],
   \"blog\":{\"id\":999,\"name\":\"Custom blog\"},\
   "author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"

To reproduce prior to this PR:
SEED=55284 rake
  1) Failure:
  ActionController::Serialization::ExplicitSerializerTest#test_render_using_explicit_each_serializer
  [active_model_serializers/test/action_controller/explicit_serializer_test.rb:139]:
  --- expected
  +++ actual
  @@ -1 +1 @@
  -"{\"id\":1,\"title\":\"New
  Post\",\"body\":\"Body\",\"comments\":[{\"id\":1,\"body\":\"ZOMG A
  COMMENT\"}],\"blog\":{\"id\":999,\"name\":\"Custom
  blog\"},\"author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"
  +"{\"id\":\"1\",\"type\":\"posts\",\"attributes\":{\"title\":\"New
  Post\",\"body\":\"Body\"},\"comments\":[{\"id\":1,\"body\":\"ZOMG A
  COMMENT\"}],\"blog\":{\"id\":999,\"name\":\"Custom
  blog\"},\"author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"

  137 runs, 211 assertions, 1 failures, 0 errors, 0 skips
  rake aborted!
  Command failed with status (1): [ruby -I"lib:test"
  -r./test/test_helper.rb
  "/$HOME/.rvm/rubies/ruby-2.2.2/lib/ruby/2.2.0/rake/rake_test_loader.rb"
  "test/action_controller/adapter_selector_test.rb"
  "test/action_controller/explicit_serializer_test.rb"
  "test/action_controller/json_api_linked_test.rb"
  "test/action_controller/rescue_from_test.rb"
  "test/action_controller/serialization_scope_name_test.rb"
  "test/action_controller/serialization_test.rb"
  "test/adapter/fragment_cache_test.rb"
  "test/adapter/json/belongs_to_test.rb"
  "test/adapter/json/collection_test.rb"
  "test/adapter/json/has_many_test.rb"
  "test/adapter/json_api/belongs_to_test.rb"
  "test/adapter/json_api/collection_test.rb"
  "test/adapter/json_api/has_many_embed_ids_test.rb"
  "test/adapter/json_api/has_many_explicit_serializer_test.rb"
  "test/adapter/json_api/has_many_test.rb"
  "test/adapter/json_api/has_one_test.rb"
  "test/adapter/json_api/linked_test.rb" "test/adapter/json_test.rb"
  "test/adapter/null_test.rb" "test/adapter_test.rb"
  "test/array_serializer_test.rb" "test/serializers/adapter_for_test.rb"
  "test/serializers/associations_test.rb"
  "test/serializers/attribute_test.rb"
  "test/serializers/attributes_test.rb" "test/serializers/cache_test.rb"
  "test/serializers/configuration_test.rb"
  "test/serializers/fieldset_test.rb"
  "test/serializers/generators_test.rb" "test/serializers/meta_test.rb"
  "test/serializers/options_test.rb"
  "test/serializers/serializer_for_test.rb"
  "test/serializers/urls_test.rb" ]
  /$HOME/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in
  `eval'
  /$HOME/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in
  `<main>'
  Tasks: TOP => default => test
  (See full trace by running task with --trace)

test/action_controller/explicit_serializer_test.rb

@@ -57,11 +57,10 @@ module ActionController
         end
 
         def render_using_explicit_each_serializer
-          @comment = Comment.new({ id: 1, body: 'ZOMG A COMMENT' })
-          @author  = Author.new(id: 1, name: 'Joao Moura.')
-          @post    = Post.new({ id: 1, title: 'New Post', body: 'Body', comments: [@comment], author: @author })
+          location       = Location.new(id: 42, lat: '-23.550520', lng: '-46.633309')
+          place          = Place.new(id: 1337, name: 'Amazing Place', locations: [location])
 
-          render json: @post, each_serializer: PostSerializer
+          render json: place, each_serializer: PlaceSerializer
         end
       end
 
@@ -118,25 +117,19 @@ module ActionController
         get :render_using_explicit_each_serializer
 
         expected = {
-          id: 1,
-          title: 'New Post',
-          body: 'Body',
-          comments: [
+          id: 1337,
+          name: "Amazing Place",
+          locations: [
             {
-              id: 1,
-              body: 'ZOMG A COMMENT' }
-          ],
-          blog: {
-            id: 999,
-            name: 'Custom blog'
-          },
-          author: {
-            id: 1,
-            name: 'Joao Moura.'
-          }
+              id: 42,
+              lat: "-23.550520",
+              lng: "-46.633309",
+              place: "Nowhere" # is a virtual attribute on LocationSerializer
+            }
+          ]
         }
 
-        assert_equal expected.to_json, @response.body
+        assert_equal expected.to_json, response.body
       end
     end
   end