Including one more spec for verifying proxy to make sure allow/receive expectations work

This is a complement to #623, this validates that:

```ruby
allow(object).to receive(:implemented) { :value }
```

Correctly evaluates the block given as the implementation.

As for the other two methods, `add_simple_stub` doesn't take a block (it prints a warning if you give it a block as it is used only at the `receive_messages` call) and `add_message_expectation` is exercised by the original fix for #623.