support query filters with urlsafe() ndb.Keys

Enables the `filters` parameter of the datastore input_readers to supply a `urlsafe()` copy of an `ndb.Key` for a `ndb.KeyProperty`.

This support is important for cases where `ndb.Key` is not properly being "JSONified," as it is transformed through pipelines.

An alternative user-end fix is to add `ndb.Key` JSONification through monkeypatching. This provides a more 'out of the box' solution for this seemingly common problem.