Require json library for to_json method

lib/oscal/assembly.rb

@@ -1,6 +1,7 @@
 require_relative "parsing_functions"
 require_relative "logger"
 require_relative "metadata_block"
+require "json"
 
 module Oscal
   class MetadataBlockWrapper < Oscal::MetadataBlock
@@ -32,8 +33,8 @@ def allowed_attributes
       end
     end
 
-    def to_json
-      to_h.to_json
+    def to_json(*args)
+      to_h.to_json(*args)
     end
 
     def to_h

lib/oscal/serializer.rb

@@ -1,4 +1,5 @@
 require "yaml"
+require "json"
 
 module Oscal
   module Serializer

spec/oscal/assessment_plan_spec.rb

@@ -15,7 +15,7 @@
 
   describe "#to_json" do
     it "generates a json representation of the assessment plan" do
-      expect(subject.to_json).to be_kind_of(String)
+      expect(subject.to_json).to eq(JSON.generate(fields))
     end
   end
 end