STIX objects with an optional "values" property can cause crash

[chisholm]

There is a workaround which attempts to allow the values attribute on STIX object instances, whose name collides with the standard python Mapping method, to work either as a property access or an invocation of the Mapping method. It was used with the WindowsRegistryKey SCO. Unfortunately, that property is optional and accessed without checking whether it exists, which means when it doesn't exist, invocation as the Mapping method raises a KeyError.