Do not emit name mutation for underscored optional arguments

Closes #98

lib/mutant/mutator/node/argument.rb

@@ -21,7 +21,6 @@ module Mutant
         # @api private
         #
         def dispatch
-          return if skip?
           emit_name_mutation
         end
 
@@ -32,6 +31,7 @@ module Mutant
         # @api private
         #
         def emit_name_mutation
+          return if skip?
           Mutator::Util::Symbol.each(name, self) do |name|
             emit_name(name)
           end

spec/unit/mutant/mutator/node/define/mutation_spec.rb

@@ -79,6 +79,21 @@ describe Mutant::Mutator, 'def' do
     it_should_behave_like 'a mutator'
   end
 
+  context 'with optional argument beginning in an underscore' do
+    let(:source) { 'def foo(_unused = true); end' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'def foo(_unused = nil); end'
+      mutations << 'def foo(_unused = false); end'
+      mutations << 'def foo(_unused = true); raise; end'
+      mutations << 'def foo(_unused); end'
+      mutations << 'def foo; end'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
   context 'default argument' do
     let(:source) { 'def foo(a = true); end' }