Splitby fix

Before, `splitBy` would drop a trailing empty string if the delimiter appeared at the end of the stream:
```javascript
'hello\nworld\n'.split('\n'); // [ 'hello', 'world', '' ]
_(['hello', '\n', 'world', '\n']).split().toArray(function (xs) { console.log(xs) }); // ['hello', 'world']
```

This fixes it so:
```javascript
_(['hello', '\n', 'world', '\n']).split().toArray(function (xs) { console.log(xs) }); // ['hello', 'world', '']
```