Move 'extensions' property to custom Observables

... from custom Observable extensions (an extension doesn't need an
'extensions' property).

stix2/test/test_custom.py

@@ -602,3 +602,8 @@ def test_register_custom_object():
     stix2._register_type(CustomObject2)
     # Note that we will always check against newest OBJ_MAP.
     assert (CustomObject2._type, CustomObject2) in stix2.OBJ_MAP.items()
+
+
+def test_extension_property_location():
+    assert 'extensions' in stix2.v20.observables.OBJ_MAP_OBSERVABLE['x-new-observable']._properties
+    assert 'extensions' not in stix2.v20.observables.EXT_MAP['domain-name']['x-new-ext']._properties

stix2/v20/observables.py

@@ -979,6 +979,9 @@ def CustomObservable(type='x-custom-observable', properties=None):
                                      "is not a ListProperty containing ObjectReferenceProperty." % prop_name)
 
             _properties.update(properties)
+            _properties.update([
+                ('extensions', ExtensionsProperty(enclosing_type=_type)),
+            ])
 
             def __init__(self, **kwargs):
                 _Observable.__init__(self, **kwargs)
@@ -1030,9 +1033,7 @@ def CustomExtension(observable=None, type='x-custom-observable', properties=None
         class _Custom(cls, _Extension):
 
             _type = type
-            _properties = {
-                'extensions': ExtensionsProperty(enclosing_type=_type),
-            }
+            _properties = OrderedDict()
 
             if not properties or not isinstance(properties, list):
                 raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")