Add some failing tests around has_many assocs...

..where no serializer is defined for the thing that is has_many'd

test/fixtures/poro.rb

@@ -76,6 +76,7 @@ Role     = Class.new(Model)
 User     = Class.new(Model)
 Location = Class.new(Model)
 Place    = Class.new(Model)
+Tag      = Class.new(Model)
 Comment  = Class.new(Model) do
   # Uses a custom non-time-based cache key
   def cache_key
@@ -224,6 +225,12 @@ PostPreviewSerializer = Class.new(ActiveModel::Serializer) do
   belongs_to :author, serializer: AuthorPreviewSerializer
 end
 
+PostWithTagsSerializer = Class.new(ActiveModel::Serializer) do
+  attributes :id
+
+  has_many :tags
+end
+
 Spam::UnrelatedLinkSerializer = Class.new(ActiveModel::Serializer) do
   attributes :id
 end