Undefine methods before loading mutants

* Closes #163

lib/mutant/loader.rb

@@ -15,6 +15,7 @@ module Mutant
       # @api private
       #
       def call
+        subject.prepare
         eval(
           source,
           TOPLEVEL_BINDING,

lib/mutant/subject.rb

@@ -29,6 +29,16 @@ module Mutant
       context.source_path
     end
 
+    # Prepare the subject for the insertion of mutation
+    #
+    # @return [self]
+    #
+    # @api private
+    #
+    def prepare
+      self
+    end
+
     # Return source line
     #
     # @return [Fixnum]

lib/mutant/subject/method.rb

@@ -27,6 +27,17 @@ 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]

spec/unit/mutant/loader/eval_spec.rb

@@ -14,6 +14,10 @@ describe Mutant::Loader::Eval, '.call' do
     double('Subject', source_path: path, source_line: line)
   end
 
+  before do
+    expect(mutation_subject).to receive(:prepare).and_return(mutation_subject)
+  end
+
   let(:source) do
     <<-RUBY
       class SomeNamespace

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

@@ -12,6 +12,28 @@ describe Mutant::Subject::Method::Instance do
     s(:def, :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 foo
+        end
+      end
+    end
+
+    subject { object.prepare }
+
+    it 'undefines method on scope' do
+      expect { subject }.to change { scope.instance_methods.include?(:foo) }.from(true).to(false)
+    end
+
+    it_should_behave_like 'a command method'
+  end
+
   describe '#source' do
     subject { object.source }