prevent quoted nulls ("null") when to_json is called on nil relationship

lib/dm-serializer/to_json.rb

@@ -41,7 +41,10 @@ def as_json(options = {})
       if options[:relationships]
         options[:relationships].each do |relationship_name, opts|
           if respond_to?(relationship_name)
-            result[relationship_name] = __send__(relationship_name).to_json(opts.merge(:to_json => false))
+            relationship = __send__(relationship_name)
+            result[relationship_name] = relationship ?
+                                        relationship.to_json(opts.merge(:to_json => false)) :
+                                        nil
           end
         end
       end