Set the external encoding to ASCII-8BIT in Extractor#extract

Suggested by the following comment:

https://github.com/ranguba/chupa-text/pull/1#discussion_r9862882
> It is nor right location to change encoding.
> Formatter should handle input as is.

> We should set right encoding before passing data to a formatter.
> Extractor#extract will be a good location to set encoding for ASCII-8BIT
data.

TODO:
  * We may not want to validate the encoding.
  * We'll make tests clear and readable.