Fix multiple diff reporter errors on memoizers

Closes #130

lib/mutant/subject/method/instance.rb

@@ -30,6 +30,17 @@ module Mutant
         class Memoized < self
           include NodeHelpers
 
+          # Return source
+          #
+          # @return [String]
+          #
+          # @api private
+          #
+          def source
+            Unparser.unparse(memoizer_node(node))
+          end
+          memoize :source
+
         private
 
           # Return mutations

spec/unit/mutant/subject/method/instance_spec.rb

@@ -0,0 +1,35 @@
+require 'spec_helper'
+
+describe Mutant::Subject::Method::Instance do
+  include Mutant::NodeHelpers
+
+  let(:object)  { described_class.new(context, node) }
+  let(:context) { double }
+
+  let(:node) do
+    s(:def, :foo, s(:args))
+  end
+
+  describe '#source' do
+    subject { object.source }
+
+    it { should eql("def foo\nend") }
+  end
+end
+
+describe Mutant::Subject::Method::Instance::Memoized do
+  include Mutant::NodeHelpers
+
+  let(:object)  { described_class.new(context, node) }
+  let(:context) { double }
+
+  let(:node) do
+    s(:def, :foo, s(:args))
+  end
+
+  describe '#source' do
+    subject { object.source }
+
+    it { should eql("def foo\nend\nmemoize(:foo)") }
+  end
+end