Throw a 401 on revoke if no auth header is found

This should fix #505

@tute I wasn't sure the best way to raise a 401 in doorkeeper. I did come across ```Doorkeeper::OAuth::ErrorResponse```, but I wasn't sure if you wanted me to do something like...
```render json: {}, status: Doorkeeper::OAuth::ErrorResponse.new.status and return unless doorkeeper_token```

Let me know if you have a better idea/way to do this.
I'd write this as:

```ruby
if doorkeeper_token.nil?
  render nothing: true, status: 401
  return
end
```