Call PartialUpdateElement but correct payload

Complements #222 (closed)

This endpoint helper was incomplete as many fields were missing and the payload sent to the server was not right. PartialUpdateElement should be called only with the fields to update. All others were overwritten.

arkindex_worker/worker/element.py

@@ -343,7 +343,8 @@ class ElementMixin(object):
                 payload["polygon"] = str(payload["polygon"])
 
             # Retrieve the right image
-            payload["image"] = CachedImage.get_by_id(payload["image"])
+            if "image" in payload:
+                payload["image"] = CachedImage.get_by_id(payload["image"])
 
             CachedElement.update(**payload).where(
                 CachedElement.id == element.id