Dont set filterer to non-function

I ran into this recently:

```coffeescript
class FilterBox extends View
  events:
    'change input': 'filter'
```

If the first argument (in this case, a jQuery event) is not a function, Chaplin incorrectly sets the `@filterer` to be that argument (even in the case where the argument is `undefined`). Particularly for non-coffeescript files, this looks really ugly. Calling `@filter()` should just work.