Add test for addition fix

Author: rmosolgo

spec/graphql/schema/addition_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+require "spec_helper"
+
+describe GraphQL::Schema::Addition do
+  it "handles duplicate types with cycles" do
+    duplicate_types_schema = Class.new(GraphQL::Schema)
+    duplicate_types = 2.times.map {
+      Class.new(GraphQL::Schema::Object) do
+        graphql_name "Thing"
+        field :thing, self
+      end
+    }
+    duplicate_types_schema.orphan_types(duplicate_types)
+    assert_equal 2, duplicate_types_schema.send(:own_types)["Thing"].size
+  end
+end