Equal to matcher no longer differentiates between different mocks of the same class

[ezzatron]

Because comparison used to descend into $_proxy inside each mock instance, and each proxy's state includes a unique integer label, this used to cause each mock instance to compare differently in cases like this:

$mockA = mock('ClassName')->mock();
$mockB = mock('ClassName')->mock();

$stub->with($mockA)->returns('a');
$stub->with($mockB)->returns('b');

But due to the changes in #82, these mock instances become equal, and it's breaking lots of existing tests.

There are a few options here:

1. The tests could be updated to use some ===-style matcher:

$stub->with(sameInstance($mockA))->returns('a');
$stub->with(sameInstance($mockB))->returns('b');

1. The proxy state, or part of it, could be re-added for comparison.
2. Mock instances could be compared with ===, in contrast with every other object type.