Only using explicit namespace when deserializing key pb.

This issue surfaced during review for #282.
Adding checks for namespace in datastore helpers test.

This is to ensure that values are correctly ignored when
the field isn't set on the protobuf.
Merge pull request #292 from dhermes/unmarshall-key-namespace

Only using explicit namespace when deserializing key pb.
Makes operator checking deterministic in Query.filter.

The order of dictionary keys is not guaranteed between runs of
code so `expression`s ending in '>=' or '<=' were sometimes
matching for '='.
Adding test to make sure the correct operators get set on filter.