dayOfWeek initialized with 0 instead of 0xFF

Reported for Version 1.0.178.

In the GXDateTime class, the variable self.dayOfWeek is initialized with "0" -> https://github.com/Gurux/Gurux.DLMS.Python/blame/9575fe600e643b0f43385d…

If a meter sends 0xFF for the dayOfWeek this value will not be updated and the resulting GXDateTime object will have "0" instead of "0xFF" for the dayOfWeek property -> https://github.com/Gurux/Gurux.DLMS.Python/blob/9575fe600e643b0f43385db…

In the C# code however DayOfWeek is initialized with 0xFF and therefor does not have this problem -> https://github.com/Gurux/Gurux.DLMS.Net/blob/446e1914e1c6d06061c190e2f1…

So self.dayOfWeek should also be initialized with 0xFF in the python GXDateTime class.