The default value for the payload parameter is not supported in the will_set method

[jimbobbennett]

To reproduce:

Call the will_set method and leave the payload parameter as its default value of None.

This causes an exception of Message can not be none.

If the payload can't be None, then it should not have a default value of None - all default values should work as their existence implies that the value is allowed.

[2bndy5]

To be clear, are we expecting to mimic paho_mqtt behavior? In that case, a payload of None would be replaced with a zero-length string as described in will_set() of the paho_mqtt docs.

[brentru]

@2bndy5 @jimbobbennett Yes, I'd like to be in-like with paho mqtt as much as possible in this library.

[brentru]

fixed by #47