Set Username/Password flags on CONNECT

The contents of username and password for the CONNECT packet were
being set but the flags to indicate they were being used was not
which meant the username and password fields were not being encoded
in the CONNECT packet.

Author: Al S-M

message.go

@@ -87,9 +87,11 @@ func newConnectMsgFromOptions(options ClientOptions) *ConnectPacket {
 	}
 
 	if options.username != "" {
+		m.UsernameFlag = true
 		m.Username = options.username
 		//mustn't have password without user as well
 		if options.password != "" {
+			m.PasswordFlag = true
 			m.Password = []byte(options.password)
 		}
 	}