Implement `respond_to?` on the example proxy.

This makes the object a better behaved object
that accurately reports what it responds to.

It's also necessary for https://github.com/rspec/rspec-expectations/pull/513
Merge pull request #651 from rspec/fix-any-instance-partial-doubles

Fix `any_instance` so that it updates already stubbed instances
Make proxies notify any instance recorders when they receive messages