Add mutations from to_{s,i,a} -> to_{str,int,ary}

[closes #217]

lib/mutant/mutator/node/send.rb

@@ -17,6 +17,9 @@ module Mutant
           send:         [:public_send],
           gsub:         [:sub],
           eql?:         [:equal?],
+          to_s:         [:to_str],
+          to_i:         [:to_int],
+          to_a:         [:to_ary],
           :== =>        [:eql?, :equal?]
         )
 

meta/send.rb

@@ -22,6 +22,33 @@ Mutant::Meta::Example.add do
   mutation 'each'
 end
 
+Mutant::Meta::Example.add do
+  source 'foo.to_s'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_s'
+  mutation 'foo.to_str'
+end
+
+Mutant::Meta::Example.add do
+  source 'foo.to_a'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_a'
+  mutation 'foo.to_ary'
+end
+
+Mutant::Meta::Example.add do
+  source 'foo.to_i'
+
+  singleton_mutations
+  mutation 'foo'
+  mutation 'self.to_i'
+  mutation 'foo.to_int'
+end
+
 Mutant::Meta::Example.add do
   source 'foo == bar'