KeyError on initialising YoutubeDL in python3 #3910

Proposed fix to Issue #3910. Also fixed a small typo in the warning message.

Here is the output when the patch is applied:

```bash
ubuntu@localhost:~$ LC_ALL=ascii python3
Python 3.4.0 (default, Apr 11 2014, 13:05:11) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from youtube_dl import YoutubeDL
>>> import sys
>>> sys.getfilesystemencoding()
'ascii'
>>> yt = YoutubeDL()
WARNING: Assuming --restrict-filenames since file system encoding cannot encode all charactes. Set the LC_ALL environment variable to fix this.
>>> 
```