Add kwbegin node as null mutator

lib/mutant/constants.rb

@@ -61,6 +61,6 @@ module Mutant
     :xstr, :def, :defs, :case, :when, :ivar, :lvar, :cvar, :gvar,
     :back_ref, :const, :nth_ref, :class, :sclass, :yield,
     :match_with_lvasgn, :match_current_line, :irange, :erange,
-    :or_asgn
+    :or_asgn, :kwbegin
   ].to_set.freeze
 end # Mutant,

lib/mutant/mutator/node/noop.rb

@@ -16,7 +16,7 @@ module Mutant
           :ivar, :restarg, :casgn, :masgn, :resbody, :retry, :arg_expr,
           :kwrestarg, :kwoptarg, :kwarg, :undef, :module, :cbase, :empty,
           :alias, :for, :xstr, :back_ref, :nth_ref, :class, :sclass, :match_with_lvasgn,
-          :match_current_line, :or_asgn
+          :match_current_line, :or_asgn, :kwbegin
         )
 
       private