More precise "created" keyword argument signals

If a document has a user given id value, the post_save signal always got the
"created" keyword argument with False value (unless force_insert is True).

This patch uses the result of getlasterror to check whether the save was an
update or not.

mongoengine/document.py

@@ -233,13 +233,24 @@ class Document(BaseDocument):
                     actual_key = self._db_field_map.get(k, k)
                     select_dict[actual_key] = doc[actual_key]
 
+                def is_new_object(last_error):
+                    if last_error is not None:
+                        updated = last_error.get("updatedExisting")
+                        if updated is not None:
+                            return not updated
+                    return created
+
                 upsert = self._created
                 if updates:
-                    collection.update(select_dict, {"$set": updates},
-                        upsert=upsert, safe=safe, **write_options)
+                    last_error = collection.update(select_dict,
+                        {"$set": updates}, upsert=upsert, safe=safe,
+                        **write_options)
+                    created = is_new_object(last_error)
                 if removals:
-                    collection.update(select_dict, {"$unset": removals},
-                        upsert=upsert, safe=safe, **write_options)
+                    last_error = collection.update(select_dict,
+                        {"$unset": removals}, upsert=upsert, safe=safe,
+                        **write_options)
+                    created = created or is_new_object(last_error)
 
             warn_cascade = not cascade and 'cascade' not in self._meta
             cascade = (self._meta.get('cascade', True)