Fix begin mutator to visit children

lib/mutant/mutator/node/begin.rb

@@ -21,7 +21,8 @@ module Mutant
               emit_self(*children)
             end
           end
-          children.each do |child|
+          children.each_with_index do |child, index|
+            mutate_child(index)
             emit(child)
           end
         end

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

@@ -191,6 +191,8 @@ describe Mutant::Mutator, 'send' do
           mutations = []
           mutations << 'foo'
           mutations << 'left - right'
+          mutations << 'left / foo'
+          mutations << 'right / foo'
         end
 
         it_should_behave_like 'a mutator'