Add a test for to_json

Author: Shuhei Kitagawa

spec/lib/multi_to_json_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe FastJsonapi::MultiToJson do
+  include_context 'movie class'
+
+  describe 'self.to_json' do
+    subject { FastJsonapi::MultiToJson.to_json movie }
+
+    it { is_expected.to eq("{\"id\":232,\"name\":\"test movie\",\"actor_ids\":[1,2,3],\"owner_id\":3,\"movie_type_id\":1}") }
+  end
+end