Handle bytes content without encoding; add regression test.

Resubmission of https://github.com/seomoz/reppy/pull/14, rebased off master.
I think something like `decode('utf-8', 'ignore')` would be sufficient. That would get it to work for everyone who's obeying the RFC, or using [Google's](https://developers.google.com/webmasters/control-crawl-index/docs/robots_txt) 'spec', which allows `utf-8`. For those who are using characters outside of `utf-8`, only the directives with such characters will be wonky.