SPARK-2110: Don't escape nicknames
org.jivesoftware.spark.UserManager#getUserNicknameFromJID(org.jxmpp.jid.BareJid) values should not be JID escaped, as that will mangle their content. The implementation of that method already appropriately escapes any JID parts that are used in the nickname.
SPARK-2110: Add missing '@' in result of unescaping JIDs
The implementation that unescapes JIDs unintentionally removes the '@' character (separating node- and domain-part). This commit restores that character.
