Passes charset from url_fix() to to_native()

Fixes Issue #582.

`url_fix` function defaults to a charset of utf-8 (which in my opinion is the way to go) but wasn't passing this charset argument to `to_native()` function, whose charset defaults to `sys.getdefaultencoding()` (problematic in python2 since essentially will fail with all non-ascii chars).

This fix simple passes the charset to to_native and the problem seems to be solved.