Can not update documents with type of pk=int and value 0

[michal-szostak]

Documents with pk type defined as fields.IntField(primary_key=True) fail to be updated giving following error:

Traceback (most recent call last):
  File "mongoengine_bug.py", line 12, in <module>
    doc.update(set__field='changed field')
  File "/usr/local/lib/python2.7/site-packages/mongoengine/document.py", line 474, in update
    'attempt to update a document not yet saved')
mongoengine.errors.OperationError: attempt to update a document not yet saved

Here is a self contained code which can reproduce this bug:

import mongoengine

mongoengine.connect('test_db')

class TestDocument(mongoengine.Document):
    id = mongoengine.fields.IntField(primary_key=True)
    field = mongoengine.fields.StringField()

doc = TestDocument(id=0, field='')
doc.save()

doc.update(set__field='changed field')

It seems that mongoengine/document.py line 466 is at fault:

 if not self.pk:

So if Document's id is 0 it is evaluated to false and throws exception
I think it should be changed to:

if self.pk is None: