Allow passing procs with variable arguments when declaring an attribute

lib/fast_jsonapi/attribute.rb

@@ -11,7 +11,7 @@ module FastJsonapi
     def serialize(record, serialization_params, output_hash)
       if include_attribute?(record, serialization_params)
         output_hash[key] = if method.is_a?(Proc)
-          method.arity == 1 ? method.call(record) : method.call(record, serialization_params)
+          method.arity.abs == 1 ? method.call(record) : method.call(record, serialization_params)
         else
           record.public_send(method)
         end
@@ -26,4 +26,4 @@ module FastJsonapi
       end
     end
   end
-end
+end

spec/lib/object_serializer_class_methods_spec.rb

@@ -230,6 +230,22 @@ describe FastJsonapi::ObjectSerializer do
         expect(serializable_hash[:data][:attributes][:title_with_year]).to eq "#{movie.name} (#{movie.release_year})"
       end
     end
+
+    context 'with &:proc' do
+      before do
+        movie.release_year = 2008
+        MovieSerializer.attribute :released_in_year, &:release_year
+      end
+
+      after do
+        MovieSerializer.attributes_to_serialize.delete(:released_in_year)
+      end
+
+      it 'returns correct hash when serializable_hash is called' do
+        expect(serializable_hash[:data][:attributes][:name]).to eq movie.name
+        expect(serializable_hash[:data][:attributes][:released_in_year]).to eq movie.release_year
+      end
+    end
   end
 
   describe '#link' do