Fix invalid AST generation on operator method with self as receiver

Changelog.md

@@ -1,3 +1,7 @@
+# v0.5.22 2014-06-15
+
+* Fix invalid AST generation on operator method mutation with self as receiver.
+
 # v0.5.21 2014-06-15
 
 * Readd mutation of index assignments

lib/mutant/mutator/node/send.rb

@@ -170,8 +170,9 @@ module Mutant
         #
         def emit_implicit_self
           emit_receiver(nil) if n_self?(receiver) && !(
-            KEYWORDS.include?(selector)     ||
-            OP_ASSIGN.include?(parent_type) ||
+            KEYWORDS.include?(selector)         ||
+            METHOD_OPERATORS.include?(selector) ||
+            OP_ASSIGN.include?(parent_type)     ||
             attribute_assignment?
           )
         end

meta/send.rb

@@ -235,6 +235,16 @@ Mutant::Meta::Example.add do
   mutation 'foo[]'
 end
 
+Mutant::Meta::Example.add do
+  source 'self[foo]'
+
+  singleton_mutations
+  mutation 'self[self]'
+  mutation 'self[nil]'
+  mutation 'self[]'
+  mutation 'foo'
+end
+
 Mutant::Meta::Example.add do
   source 'foo[*bar]'