Fix: use addition to increase message size
As pointed out in #6, when using a username and password to connect to a broker, the message size has been manipulated using string concatenation instead of mathematical addition. This caused an issue with chr() expecting an integer and not a string. It also would have lead to wrong information being sent.
Usernames and passwords should now be correctly encoded during the connection handshake. Thanks to @sten for pointing out this oversight!