Fix undef before insertion for singleton methods

lib/mutant/subject/method.rb

@@ -27,17 +27,6 @@ module Mutant
         node.children[self.class::NAME_INDEX]
       end
 
-      # Prepare subject for mutation insertion
-      #
-      # @return [self]
-      #
-      # @api private
-      #
-      def prepare
-        scope.send(:undef_method, name)
-        self
-      end
-
       # Return match expression
       #
       # @return [String]

lib/mutant/subject/method/instance.rb

@@ -24,6 +24,17 @@ module Mutant
         end
         memoize :public?
 
+        # Prepare subject for mutation insertion
+        #
+        # @return [self]
+        #
+        # @api private
+        #
+        def prepare
+          scope.send(:undef_method, name)
+          self
+        end
+
       private
 
         # Mutator for memoized instance methods

lib/mutant/subject/method/singleton.rb

@@ -24,6 +24,17 @@ module Mutant
         end
         memoize :public?
 
+        # Prepare subject for mutation insertion
+        #
+        # @return [self]
+        #
+        # @api private
+        #
+        def prepare
+          scope.singleton_class.send(:undef_method, name)
+          self
+        end
+
       end # Singleton
     end # Method
   end # Subject

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

@@ -0,0 +1,42 @@
+# encoding: UTF-8
+
+require 'spec_helper'
+
+describe Mutant::Subject::Method::Singleton do
+  include Mutant::NodeHelpers
+
+  let(:object)  { described_class.new(context, node) }
+  let(:context) { double }
+
+  let(:node) do
+    s(:defs, s(:self), :foo, s(:args))
+  end
+
+  describe '#prepare' do
+
+    let(:context) do
+      Mutant::Context::Scope.new(scope, double('Source Path'))
+    end
+
+    let(:scope) do
+      Class.new do
+        def self.foo
+        end
+      end
+    end
+
+    subject { object.prepare }
+
+    it 'undefines method on scope' do
+      expect { subject }.to change { scope.methods.include?(:foo) }.from(true).to(false)
+    end
+
+    it_should_behave_like 'a command method'
+  end
+
+  describe '#source' do
+    subject { object.source }
+
+    it { should eql("def self.foo\nend") }
+  end
+end