Fix encoding for m3u links, again

[RomanPro100]

PR #340 was supposed to solve issue #339, but it caused a bigger problem - some new providers could not be added (#365).

Both of these issues have to do with what encodings providers use - some use None encoding, some have apparent_encoding different from the actual encoding:

Playlist links: test1 and test2

In this PR both of the issues are fixed by specifying the response encoding if it's None and using this encoding when downloading playlists.

[gena01]

Ran into the issue #365 recently, and this fixes the loading of channels for me.

P.S. Because of bug #343 it's harder to verify, but if you are adding a new provider/url you see no data being loaded without this fix.