IPFS Gateway: directory URL should end with slash

Bugs
1

Relative paths are tricky to get right on IPFS gateway.
To remove the most common pitfail, go-ipfs returns HTTP 302 redirect when the root directory is requested without trailing slash and adds it automatically:

$  curl -I https://ipfs.io/ipfs/QmaXPQ28xM7h8AjnbJ8cD6Ux4mgvqtwGRYjgsxEPQ6kJTX
HTTP/2 302
...
location: /ipfs/QmaXPQ28xM7h8AjnbJ8cD6Ux4mgvqtwGRYjgsxEPQ6kJTX/

Unfortunately Infura disabled this behavior and returns HTML immediately for URLs without trailing slash:

This is error prone, moves the base URL from /ipfs/QmaXPQ28xM7h8AjnbJ8cD6Ux4mgvqtwGRYjgsxEPQ6kJTX/ to /ipfs/, breaking relative paths as described here.

Restoring the redirect should fix all websites with relative paths.