oak's path traversal allows transfer of hidden files within the served root directory

`oak` is a middleware framework for Deno's native HTTP server, Deno Deploy, Node.js 16.5 and later, Cloudflare Workers and Bun. By default `oak` does not allow transferring of hidden files with `Context.send` API. However, prior to version 17.1.3, this can be bypassed by encoding `/` as its URL encoded form `%2F`. For an attacker this has potential to read sensitive user data or to gain access to server secrets. Version 17.1.3 fixes the issue.