Fixes double encoding issue presented with the 0.8.7 release

The 0.8.7 release causes a new issue when using the `list` function with params.prefix, when prefix has special characters in it (like slashes).  This causes lists to fail in those cases because the prefix is mangled.

```
prefix: 'foo/bar-wildcard',
delimiter: '/'
```
This changed the GET that looked like this in <= 0.8.6:
`/?prefix=foo%2Fbar-wildcard&delimiter=%2F`

To the GET that looked like this in 0.8.7:
`/?prefix=foo%252Fbar-wildcard&delimiter=%252F`

The fix is to pass through the qs.stringify result as an unencoded query string, so that when `request` picks it up, it encodes it properly, as opposed to double-encoding.