Fixed encoding bug with blackhole downloader

The blackhole downloader can return a ```UnicodeEncodeError``` when saving some torrents. I narrowed this issue down to an encoding issue as a result of using the ```.text``` attribute from the download response when ```.content``` should be used (```return_raw = True``` corrects this).

**Note**
This could possibly break other downloaders as the ```filedata``` encoding might not be accepted by some of the downloaders. Do we need to test this on each downloader? or is there a better way we could handle this?