[imap-server] Outlook client uses obsolete RFC822.peek in fetch calls

[dejanstrbac]

Outlook issues request for RFC822.peek which is obsoleted by [RFC2060] and not implemented in go-imap

7 UID fetch 39 (UID RFC822.SIZE RFC822.peek)
7 BAD [CLIENTBUG] Unknown FETCH data item

https://learn.microsoft.com/en-us/openspecs/exchange_standards/ms-stanoimap/8e9492b1-b812-4a1a-93ea-4d69489eb5cd

[emersion]

This should help: #658