Add missing masgn mutations

Author: mbj

lib/mutant/mutator/node/masgn.rb

@@ -15,6 +15,7 @@ class MultipleAssignment < self
 
         def dispatch
           emit_singletons
+          emit_right_mutations
         end
 
       end # MultipleAssignment

meta/masgn.rb

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
 Mutant::Meta::Example.add :masgn do
-  source 'a, b = c, d'
+  source 'a, b = nil, nil'
 
   singleton_mutations
+
+  mutation 'a, b = nil'
+  mutation 'a, b = []'
+  mutation 'a, b = [nil]'
 end