Choose key without fragment if key with fragment is not found

[dahlia]

Context: fedify-dev/hollo#109 (comment).

According to ActivityPub and HTTP Signatures:

Extract the signature's keyId parameter. It should be a URL. If it has a fragment, discard it.

Accordance with the robustness principle—be conservative in what you do, be liberal in what you accept from others, Fedify should do its best to find the key.

[dahlia]

Discarding a fragment should be done only when there is only one public key. Otherwise, it should work as it is.