Function peer_allows 

pub fn peer_allows(headers: &Headers) -> bool

Whether a peer’s Allow lists UPDATE (RFC 3311 §4).

Absent means no. §4 is a SHOULD on the sender, so a peer that supports UPDATE and does not say so is indistinguishable from one that does not support it — and guessing wrong turns a session refresh into a request the far end answers 405, which is a call torn down for a capability nobody needed.