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.