Correct ldap username validation. (#2880)

PR #342 was only partially applied. Spaces should not be at the start
and end of a username but they can be inside.

Author: Jochen Rill

modules/auth/ldap/ldap.go

@@ -69,7 +69,7 @@ func (ls *Source) sanitizedUserQuery(username string) (string, bool) {
 
 func (ls *Source) sanitizedUserDN(username string) (string, bool) {
 	// See http://tools.ietf.org/search/rfc4514: "special characters"
-	badCharacters := "\x00()*\\,='\"#+;<> "
+	badCharacters := "\x00()*\\,='\"#+;<>"
 	if strings.ContainsAny(username, badCharacters) {
 		log.Debug("'%s' contains invalid DN characters. Aborting.", username)
 		return "", false