Ensure that nnimap doesn't send too long lines to the server

* lisp/gnus/nnimap.el (nnimap-retrieve-headers): Don't send
too-long lines to the server (bug#56138).
1 job for master in 2 minutes and 17 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Normal
passed #48080
test-lisp-gnus-inotify

00:02:17