Only call _invalidate_all if it exists

the memoize decorator adds the _invalidate_all function when called.
Due to the deferred initialization that may not happen in a test.

tests/api_tests.py

@@ -48,6 +48,7 @@ class TestApiCalls(unittest.TestCase):
 
     def tearDown(self):
         """Clean internal cache"""
+        if hasattr(self.api, '_invalidate_all'):
             self.api._invalidate_all()
 
     def test_ring_packages(self):