Fix Emacs28 compatibility and issue with default imap-user nil (#14)

[stasvlasov]

Fixes issues described in #14

The problem with loading token info from file (i.e. auth-source-xoauth2-creds is a string with file name) can be solved by adding user name/login to this file and setting user as nil in the hash-table because it seems that nnimap-credentials do not provide :user to the auth-source-search (just the default imap-user which is set to nil). The auth-source-xoauth2-creds could be then something like:

#s(hash-table size 2 test equal
data (("host1.com" nil "port1")
(:token-url "token-url-1"
:client-id "client-id-1"
:client-secret "client-secret-1"
:refresh-token "refresh-token-1"
:user "user-1")

         (\"host2.com\" nil \"port2\")
         (
          :token-url \"token-url-2\"
          :client-id \"client-id-2\"
          :client-secret \"client-secret-2\"
          :refresh-token \"refresh-token-2\"
          :user \"user-2\")))