Don't include empty polymoprhic associations

Take this serializer:

class TodoSerializer < ActiveModel::Serializer
  root :todo, :include => true
  has_one :reference, :polymorphic => true
end

A nil reference would generate this JSON:

{
  "todo": { "reference": null },
  "nil_classes": []
}

This commit prevents the `nil_classes` key from being added when
serializing and including nil polymoprhic associations.

test/serializer_test.rb

@@ -1244,4 +1244,24 @@ class SerializerTest < ActiveModel::TestCase
       smoothie.as_json
     end
   end
+
+  def tests_includes_does_not_include_nil_polymoprhic_associations
+    post_serializer = Class.new(ActiveModel::Serializer) do
+      root :post
+      embed :ids, :include => true
+      has_one :author, :polymorphic => true
+      attributes :title
+    end
+
+    post = Post.new(:title => 'Foo')
+
+    actual = post_serializer.new(post).as_json
+
+    assert_equal({
+      :post => {
+        :title => 'Foo',
+        :author => nil
+      }
+    }, actual)
+  end
 end