fix: Avoid errors with custom object ids in the list of object refs in the Report object

stix2/properties.py

@@ -477,7 +477,7 @@ class ReferenceProperty(Property):
         if self.valid_types:
             ref_valid_types = enumerate_types(self.valid_types, 'v' + self.spec_version.replace(".", ""))
 
-            if possible_prefix in ref_valid_types:
+            if possible_prefix in ref_valid_types or self.allow_custom:
                 required_prefix = possible_prefix + '--'
             else:
                 raise ValueError("The type-specifying prefix '%s' for this property is not valid" % (possible_prefix))

stix2/v20/sdo.py

@@ -250,6 +250,7 @@ class Report(STIXDomainObject):
     ])
 
     def __init__(self, *args, **kwargs):
+        self._properties['object_refs'].contained.allow_custom = kwargs.get('allow_custom', False)
         self._properties['object_refs'].contained.interoperability = kwargs.get('interoperability', False)
 
         super(Report, self).__init__(*args, **kwargs)