cursor_after() doesn't exist at end of iteration

[oliverchang]

There is a behaviour difference with the old NDB that we're relying on wrt to cursor_after() at the end of iterating through a query.

Given the following:

      class Model(ndb.Model):                 
        a = ndb.IntegerProperty()                 
                                      
      Model(a=1).put()   
      Model(a=2).put()
      Model(a=3).put()   
                 
      result = Model.query().iter(limit=2)
      print(list(result))                                                                               
      print(list(Model.query().iter(start_cursor=result.cursor_after())))

The old NDB gives:

[Model(key=Key('Model', 31), a=1), Model(key=Key('Model', 32), a=2)]
[Model(key=Key('Model', 33), a=3)]

The new NDB gives:

[Model(key=Key(u'Model', 31L), a=1L), Model(key=Key(u'Model', 32L), a=2L)]
BadArgumentError: There is no cursor currently

[oliverchang]

This works as expected if the following line is removed:

python-ndb/google/cloud/ndb/_datastore_query.py

Line 325 in 2d8857b

self._cursor_before = self._cursor_after = None