deparam: parse params with remaining ampersand

Pretty easy fix for https://github.com/bitovi/canjs/issues/331

can.deparam is not able to parse following params 

```javascript
can.deparam("a=b&")
```

It fails with `Cannot read property 'length' of null` error.

This is pretty common case when somebody generate url wrong (in javacript, on server, or write bad url into browser)

Example http://jsfiddle.net/eRQBF/