method_receiver returns inconsistent receiver for self.method

[alexcrocha]

self.bar inside class Foo and Foo.bar both call a method on the Foo class object, but method_receiver returns different receivers:

  class Foo
    self.bar  # receiver: Foo
  end

  Foo.bar     # receiver: <Foo>

I would expect these to be equivalent, as both target a method on the singleton class of Foo.

Found while pairing on #645 and investigating the behaviour of method_receiver.