Assert: Allow expected to be a string in `throws`.

Prior to this the following would ignore the third param (the intended message):

```javascript
throws(
  function() {
    throw new Error('my expected error string');
  },
  'my expected error string',
  'throws the correct error');
```

The documentation states that a string value is acceptable.

![screenshot](http://monosnap.com/image/Dxav7TZEulBJ92RtZRiSh7T890kX6t.png)