Fixed error: when allow change username was set to false, registration failed.

packages/rocketchat-lib/server/methods/setUsername.coffee

@@ -3,12 +3,12 @@ Meteor.methods
 		if not Meteor.userId()
 			throw new Meteor.Error('invalid-user', "[methods] setUsername -> Invalid user")
 
-		unless RocketChat.settings.get("Accounts_AllowUsernameChange")
-			throw new Meteor.Error(403, "[methods] resetAvatar -> Invalid access")
+		user = Meteor.user()
 
-		console.log '[methods] setUsername -> '.green, 'userId:', Meteor.userId(), 'arguments:', arguments
+		if user.username? and not RocketChat.settings.get("Accounts_AllowUsernameChange")
+			throw new Meteor.Error(403, "[methods] setUsername -> Username change not allowed")
 
-		user = Meteor.user()
+		console.log '[methods] setUsername -> '.green, 'userId:', Meteor.userId(), 'arguments:', arguments
 
 		if user.username is username
 			return username