Add kwarg mutations

lib/mutant/mutator/node/argument.rb

@@ -4,7 +4,7 @@ module Mutant
 
       # Mutator for required arguments
       class Argument < self
-        handle(:arg)
+        handle(:arg, :kwarg)
 
         UNDERSCORE = '_'.freeze
 

lib/mutant/mutator/node/generic.rb

@@ -9,7 +9,7 @@ module Mutant
         # your contribution is that close!
         handle(
           :ensure, :redo, :regopt, :retry, :arg_expr, :blockarg,
-          :kwrestarg, :kwoptarg, :kwarg, :undef, :module, :empty,
+          :kwrestarg, :kwoptarg, :undef, :module, :empty,
           :alias, :for, :xstr, :back_ref, :class, :restarg,
           :sclass, :match_with_lvasgn, :while_post,
           :until_post, :preexe, :postexe, :iflipflop, :eflipflop, :kwsplat,

meta/kwarg.rb

@@ -4,4 +4,5 @@ Mutant::Meta::Example.add :kwarg do
   mutation 'def foo; end'
   mutation 'def foo(bar:); raise; end'
   mutation 'remove_method(:foo)'
+  mutation 'def foo(_bar:); end'
 end