Store Classification in local cache during create_classification

Closes #56 (closed)

arkindex_worker/worker/classification.py

@@ -88,7 +88,7 @@ class ClassificationMixin(object):
             created = self.request(
                 "CreateClassification",
                 body={
-                    "element": element.id,
+                    "element": str(element.id),
                     "ml_class": self.get_ml_class_id(None, ml_class),
                     "worker_version": self.worker_version_id,
                     "confidence": confidence,