Add mutation from send to __send__

* `__send__` is the canonical form.

lib/mutant/mutator/node/send.rb

@@ -17,7 +17,8 @@ module Mutant
           reverse_each:  [:each],
           reverse_merge: [:merge],
           map:           [:each],
-          send:          [:public_send],
+          send:          [:public_send, :__send__],
+          __send__:      [:public_send],
           gsub:          [:sub],
           eql?:          [:equal?],
           to_s:          [:to_str],

meta/send.rb

@@ -189,12 +189,26 @@ Mutant::Meta::Example.add do
   mutation 'self.gsub(a, b)'
 end
 
+Mutant::Meta::Example.add do
+  source 'foo.__send__(bar)'
+
+  singleton_mutations
+  mutation 'foo.__send__'
+  mutation 'foo.public_send(bar)'
+  mutation 'bar'
+  mutation 'foo'
+  mutation 'self.__send__(bar)'
+  mutation 'foo.__send__(nil)'
+  mutation 'foo.__send__(self)'
+end
+
 Mutant::Meta::Example.add do
   source 'foo.send(bar)'
 
   singleton_mutations
   mutation 'foo.send'
   mutation 'foo.public_send(bar)'
+  mutation 'foo.__send__(bar)'
   mutation 'bar'
   mutation 'foo'
   mutation 'self.send(bar)'