CONNACK property length should be optional

[Tatsuzo Osawa <tatsuzo.osawa@gmail.com>]

Thank you for the great work.

I have one comment for COS01.

Regarding 3.2.2.3.1 (CONNACK) Property Length,
I'd like to add "If the Remaining Length is 2 there is no Property
Length and the value of 0 is used."

Because, all other messages without payload has similar statement.
Only CONNACK has not. In addition, treating the property length as
optional helps interoperability with the former version.

Best regards,
Tatsuzo Osawa