Fix invalid block mutations on lambda

[Fix #802]

lib/mutant/mutator/node/block.rb

@@ -51,7 +51,7 @@ module Mutant
         #
         # @return [undefined]
         def mutate_body_receiver
-          return unless n_send?(body)
+          return if n_lambda?(send) || !n_send?(body)
 
           body_meta = AST::Meta::Send.new(body)
 

meta/lambda.rb

@@ -7,3 +7,17 @@ Mutant::Meta::Example.add :block, :lambda do
 
   mutation '->() { raise }'
 end
+
+Mutant::Meta::Example.add :block, :lambda do
+  source '->() { foo.bar }'
+
+  singleton_mutations
+
+  mutation '->() { }'
+  mutation '->() { self }'
+  mutation '->() { nil }'
+  mutation '->() { raise }'
+  mutation '->() { self.bar }'
+  mutation '->() { foo }'
+  mutation 'foo.bar'
+end