'&' in group names breaks nnimap

[Steve Purcell]

Given an IMAP folder called, say, "Friends.Bob & Jane", GNUs is unable
to access the contents of the folder.

In the group list, the group gets displayed as "Friends.Bob &-
Jane", and opening the group results in an error message like this:

    gnus-select-newsgroup: Couldn't request group Friends.Bob &- Jane: NO 
Mailbox doesn't exist: Friends.Bob &-- Jane

I poked around in nnimap.el, and applying utf7-decode to the group names
returned by nnimap-get-groups makes the mailboxes accessible, but
phantom copies of the groups with the escaped names still end up in the group
list after quitting and re-entering Gnus.

Is this a known issue?

(This is with the GNUs bundled in an Emacs snapshot from a few days
ago.)

-Steve