Fixed bug accessing ListField (BaseList) with negative indices

If you __setitem__ in BaseList with a negative index and then try to save this, you will get an error like: OperationError: Could not save document (cannot use the part (shape of signal.shape.-1) to traverse the element ({shape: [ 0 ]})). To fix this I rectify negative list indices in BaseList _mark_as_changed as the appropriate positive index. This fixes the above error.

mongoengine/base/datastructures.py

@@ -199,7 +199,8 @@ class BaseList(list):
     def _mark_as_changed(self, key=None):
         if hasattr(self._instance, '_mark_as_changed'):
             if key:
-                self._instance._mark_as_changed('%s.%s' % (self._name, key))
+                self._instance._mark_as_changed('%s.%s' % (self._name, 
+                    key % len(self)))
             else:
                 self._instance._mark_as_changed(self._name)