Pooled transport ignores per-message auth and sends as the pool user

Hey! Found that when using the pooled SMTP transport, a message carrying its own auth goes out over a pooled connection logged in as the pool user, so it is sent under the wrong identity with no error. The single-shot transport honors per-message auth (it merges over the transport options), the pool just never reads it. Happy to PR a fix that routes such messages through a dedicated one-shot sender if that helps.