Stop mutation of #to_i to implicit self

The mutations of all calls to `#to_i` include one that mutates the call into a
call to `Integer`. This is a great mutation in all calses where there is an
explicit receiver for the `#to_i` call. However, the way that the implicit call
to `self.to_i` is parsed results in a receiver of `nil` instead of `self`.

As such, we want to limit this particular mutation to calls to `#to_i` that have
an explicit receiver. This feels like the correct behavior because the result of
the mutation would be (if it parsed): `Integer(nil)`, which doesn't retain the
intended behavior of the original source.

Another way to approach this would be to mutate the code to `Integer(self)`, but
that doesn't feel as correct to me because if you're using the implicit `self`
with a call to `#to_i`, you likely are not implementing the strict `#to_int`
method as well, which `Integer` relies on.

This fix feels like the right mix of correctness and minimal invasiveness.

Closes #738

config/flay.yml

@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 1321
+total_score: 1316

lib/mutant/mutator/node/send.rb

@@ -182,7 +182,7 @@ module Mutant
         #
         # @return [undefined]
         def emit_integer_mutation
-          return unless selector.equal?(:to_i)
+          return unless receiver && selector.equal?(:to_i)
 
           emit(s(:send, nil, :Integer, receiver))
         end

meta/send.rb

@@ -135,6 +135,13 @@ Mutant::Meta::Example.add :send do
   mutation 'map'
 end
 
+Mutant::Meta::Example.add :send do
+  source 'to_i'
+
+  singleton_mutations
+  mutation 'to_int'
+end
+
 Mutant::Meta::Example.add :send do
   source 'foo.to_s'