Improve warning message when using `and_call_original` with a stub block...

as we can see in #774 
The problem with this warning is that there are ways you can override a stub; for example:

``` ruby
allow(k).to receive(:to_s) { "a" }.with(no_args) { "b" }
```

In this particular case, `and_call_original` isn't involved at all, but with your change the warning will improperly say that it is.

This is why the message is worded so generally...because there are many ways to get in this situation where you are overriding a previous implementation.