'withArgs' allways runs in global scope

I'm trying to do something like that:

```
expect(obj.method).withArgs(arg1).to.not.throwException();
```

And when I do it I expect that `obj.method` would be calling in `obj` context (_this_ would be a reference to `obj`).
But there is a string:

```
fn.apply(null, args);
```

in `Assertion.prototype.withArgs`. So `fn` would _always_ called in context of null, which in browser would be replacing with Window object. And I'll always get an `global leaks detected` error even if there is no actually global leaks.