Fix for donwcase issue on non-latin strings

[netoneko]

Got 2 problems with cyrillic text:

1. :remove_accents just destoys all cyrillic letters.
2. Downcase does not happen because of str.mb_chars.to_s.doncase instead of str.mb_chars.down_case.to_s.

I've fixed second problem and just avoid first with disabling remove_accents.

Probably first problem worth mentioning in Readme.md file.

[dblock]

This is great, but could you please add a test or two? Thanks.

[netoneko]

Okay, why not.

[netoneko]

By the way, I ran the tests and replaced

      if config[:remove_accents]
        str = UnicodeUtils.nfkd(str).gsub(/[^\x00-\x7F]/,'')
      end

with

      if config[:remove_accents]
        str = UnicodeUtils.nfkd(str)
      end

Got all greens. Shouldn't this gsub just be removed? In that case it won't mess with Cyrillic letters.

[dblock]

I think you're right. @aaw, want to pitch in here?

[dblock]

@netoneko Please merge from upstream, there were some changes ...

[netoneko]

Completed the merge.

[dblock]

Talked to @aaw. Merging. I am happy you've used a Russian art reference in the spec to keep things consistent :)