ShouldBeTypeOf should return the object

While writing some test, I frequently came across this pattern:

```csharp
bar.ShouldBeTypeOf<Foo>();
var baz = (Foo) bar;
// do something with baz
```

It would be great if `ShouldBeTypeOf` just would return the object casted to the correct type, so I'd only need to write

```csharp
var baz = bar.ShouldBeTypeOf<Foo>();
// do something with baz
```

Of course this will break the ABI, so a version bump or a different method name, like `ShouldBeTypeOfAndReturn` or something like that would be necessary...