Adds failing example demonstrating regression with #264
Candidate fix for #264

* Calling `Method#to_proc` binds `self` to the object which does not do
  the right thing when accessing instance variables from a child class
  (see the test case)
* Correctly restores the visibility of methods that we did not stash.
  The visibility might have changed, though, because the same method can
  have different visibilities depending on from where it is called.
* Do not attempt to restore the visibility of simple stubs because their
  visibility will not change and we did not store it in the first place.
Fixes typo in step name