make Property.__get__ return self if instance is None

[gemerden]

Currently the property descriptor cannot be called on the owner class, with error: AttributeError: 'NoneType' object has no attribute '__node__'. Calling the property on the class can be very useful for introspection. Also it is easily fixed bij changing:

class Property(object):
    def __get__(self, instance, owner):
        value = instance.__node__[self.key]
        if value is None:
            value = self.default
        return value

in:

class Property(object):
    def __get__(self, instance, owner):
        if instance is None:
            return self
        value = instance.__node__[self.key]
        if value is None:
            value = self.default
        return value

This is very common in descriptors.

[motey]

dup #784