rgw: fix decoding + characters in URL

Fixes: #8702
Backport: firefly

Only decode + characters to spaces if we're in a query argument. The +
query argument.  The + => ' ' translation is not correct for
file/directory names.

Resolves http://tracker.ceph.com/issues/8702

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>
Signed-off-by: Brian Rak <dn@devicenull.org>