Unauthorized error to_s breaks if request is nil

According to

https://github.com/oauth-xx/oauth-ruby/blob/200b109736c4ac8609e77c7b536e429e0b2522fe/lib/oauth/errors/unauthorized.rb#L2-L11

the argument `request` defaults to nil, but if called with nil, `.to_s` will break with

```ruby
     NoMethodError:
       undefined method `code' for nil:NilClass
```