Ensure Rack::Utils.best_q_match to respect requested content type

I've tried to use this method like this:

```ruby
Rack::Utils.best_q_match "application/json", Rack::Mime::MIME_TYPES.values
```

It was returning `"application/vnd.lotus-1-2-3"` instead of `"application/json"`.

Hint:
```ruby
puts Rack::Mime::MIME_TYPES.values.first # => "application/vnd.lotus-1-2-3"
```