Add mutation Hash#fetch -> Hash#key?

Fixes #457

Changelog.md

@@ -1,6 +1,7 @@
 # Master (unreleased)
 
 * Add mutation from `Hash#[]` to `Hash#key?` (#511)
+* Add mutation from `Hash#fetch` to `Hash#key?` (#511)
 
 # v0.8.8 2015-xx-xx
 

lib/mutant/mutator/node/send.rb

@@ -27,6 +27,7 @@ module Mutant
           to_a:          %i[to_ary],
           to_h:          %i[to_hash],
           at:            %i[fetch],
+          fetch:         %i[key?],
           :[] =>         %i[at fetch key?],
           :== =>         %i[eql? equal?],
           :>= =>         %i[> == eql? equal?],

meta/send.rb

@@ -359,6 +359,15 @@ Mutant::Meta::Example.add do
   mutation 'foo(nil)'
 end
 
+Mutant::Meta::Example.add do
+  source 'self.fetch(nil)'
+
+  singleton_mutations
+  mutation 'self.fetch'
+  mutation 'fetch(nil)'
+  mutation 'self.key?(nil)'
+end
+
 Unparser::Constants::KEYWORDS.each do |keyword|
   Mutant::Meta::Example.add do
     source "foo.#{keyword}(nil)"