btdigg engine: fix text mojibake.

Seeing on the official instance, searching with non-ascii character
usually renders mojibake non-sense:

    http://searx.me/?q=%E9%A3%9F%E5%A0%82&categories=files

Screenshot with broken: ![](https://imgrush.com/1NNr_Rpv-rt2.png)

Screenshot of the fixed rendering: ![](https://imgrush.com/khe4M2P3VAyq.png)