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

@@ -283,10 +283,17 @@ class ElementMixin(object):
         Updates an element through the API.
 
         :param element: The element to update.
-        :param type: Optional new slug type of the element.
-        :param name: Optional new name of the element.
-        :param polygon: Optional new polygon of the element.
-        :param confidence: Optional new confidence score, between 0.0 and 1.0.
+        :param type: Slug type of the element.
+        :param name: Name of the element.
+        :param **kwargs:
+
+            * *polygon* (``list``): Optional polygon for this element
+            * *confidence* (``float``): Optional confidence score of this element
+            * *rotation_angle* (``int``): Optional rotation angle of this element
+            * *mirrored* (``bool``): Optional mirror status of this element
+            * *image* (``UUID``): Optional ID of the image of this element
+
+
         :returns: A dict from the ``UpdateElement`` API endpoint,
         """
         assert element and isinstance(