Sunset x-ipfs-path header and unsupported characters

HTTP header values can only include characters from a limited set, notably not the full unicode smorgasbord.

The HTTP gateway spec doesn't say what to do when the `x-ipfs-path` header would end up containing these characters.

Should they be percent-escaped? Or should the header be omitted entirely?

Read the HTTP gateway specification and the existing definition of the x-ipfs-path header first. Resolve the behavior for unsupported Unicode characters, then document the chosen rule in the relevant specification section and ensure the wording covers both percent-escaping and omission cases.

Written by the indexing model from the issue text.