Fix right hand blindspot for or-assgns

Fixes #421

lib/mutant/mutator/node/send.rb

@@ -158,7 +158,6 @@ module Mutant
           emit_receiver(nil) if n_self?(receiver) && !(
             KEYWORDS.include?(selector)         ||
             METHOD_OPERATORS.include?(selector) ||
-            OP_ASSIGN.include?(parent_type)     ||
             meta.attribute_assignment?
           )
         end

meta/or_asgn.rb

@@ -21,6 +21,15 @@ Mutant::Meta::Example.add do
   mutation '@a ||= 2'
 end
 
+Mutant::Meta::Example.add do
+  source '@a ||= self.bar'
+
+  singleton_mutations
+  mutation '@a ||= nil'
+  mutation '@a ||= self'
+  mutation '@a ||= bar'
+end
+
 Mutant::Meta::Example.add do
   source 'foo[:bar] ||= 1'