Ignore new HTTP Priority header

[binwiederhier]

Cloudflare apparently adds a Priority: u=... header to requests, which ntfy rejects with 500 invalid priority (should be a 400). This is a new HTTP standard (https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority) that is emerging.

We should ignore header values that start with u=. It is quite unfortunate that the name conflicts like this.

[binwiederhier]

Fixed in 95bd876

[binwiederhier]

This ticket (#353) has a full dump of the Cloudflare request showing the Priority: u=3 header.