IMAP search should only use charset utf-8 with multibyte strings

IMAP search should only use charset utf-8 with multibyte strings

Background: Exchange servers cannot accept "charset utf-8" search
strings. The code originally set that charset for any server with
literal+ capability, borking all searches on an Exchange server. This
code only sets utf-8 for multibyte search strings in particular, which
would be borken for Exchange anyway.

* lisp/gnus/gnus-search.el (gnus-search-imap-search-command): Ensure
we're only doing the literal+ dance for multibyte strings (multibyte
strings will have had newlines inserted in an earlier step).