suggested fix for issue #1785: overloading and __set bug

[EVODelavega]

The Stub::* methods all tend to set a __mocked property on the mocked classes. When attempting to mock a class that implements a __set method that doesn't allow (as PHP calls it) "overloading", this results in a PHPUnit exception.

This PR addresses this issue by checking the class for an existing __set method. If such a method exists, the __mocked flag is not set.
This check is also used when attempting to add a non-existent property to classes with a __set method. These properties will be added, but a PHPUnit_Framework_Exception will be thrown if this fails.

The __mocked flag was primarily (only?) used in the Stub::update call (to change the mock's behavior on the fly). the __mocked property check has been replaced with an instanceof PHPUnit_Framework_MockObject_MockObject check. If this check fails, the behavior remains unchanged: calling Stub::update on anything that is not an instance of the MockObject class cause a LogicException to be thrown.

[DavertMik]

Thanks. This looks like a really serious work done to fix this issue. Probably we can remove __mocked at all. Anyway, I'm really glad to get this merged

[DavertMik]

Actually the second reason to implement __mocked was to show mocked class name in console output. Probably this can be improved as well
https://github.com/Codeception/Codeception/blob/2.0/src%2FCodeception%2FStep.php#L69

[EVODelavega]

Happy to see my PR merged, If I find the time, I'll set about removing the __mocked property entirely, and look into an alternative approach to display the mocked class name (which probably won't be that hard of a thing to do, there's probably some method on the PHPUnit_Framework_MockObject_MockObject instance I could use, or keep a reference to the ReflectionClass instance somewhere. If I get 'round too it, I'll make a PR.

PS: I think that, having merged this PR, you can close issue #1785 too