Updating fields named 'size'

Greetings!

I've greatly enjoyed using MongoEngine :) One weird thing that's happened to me is that, upon defining a document with a field named 'size,' I'm unable to use .update(set__size=value). Here's an example:

class MyDoc(mongoengine.Document):
   size = mongoengine.IntField()

mine = MyDoc()
mine.size = 42
mine.save()

MyDoc.objects.update(set__size=0)

Upon executing the above, I receive the following error:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/mongoengine/queryset/base.py", line 518, in update
    update = transform.update(queryset._document, **update)
  File "/usr/local/lib/python3.6/dist-packages/mongoengine/queryset/transform.py", line 276, in update
    field = cleaned_fields[-1]
  IndexError: list index out of range

This does not happen with Int fields named something else.

Start by reproducing the example, then inspect queryset/base.py and queryset/transform.py around the update path and the failing line 276. Confirm that updating a field named size succeeds and add a regression test covering the demonstrated MyDoc update.