If httpsig verification fails, fix Host header and try again

If httpsig verification fails, fix Host header and try again

This fixes a very rare bug when Gitea and another AP server (confirmed to happen with Mastodon) are running on the same machine, Gitea fails to verify incoming HTTP signatures. This is because the other AP server creates the sig with the public Gitea domain as the Host. However, when Gitea receives the request, the Host header is instead localhost, so the signature verification fails. Manually changing the host header to the correct value and trying the verification again fixes the bug.