Add support for explicit deserialization of NodeJS bundles

[tseaver]

Left over from PR #319. See this comment in the unit tests for bundling:

        # The following assertions would test deserialization of NodeJS bundles
        # were explicit handling of that edge case to be added.


        # First, deserialize that value into a Bundle instance. If this succeeds,
        # we're off to a good start.
        # bundle = _helpers.deserialize_bundle(_serialized, client=client)
        # Second, re-serialize it into a Python-centric format (aka, ISO timestamps)
        # instead of seconds/nanos.
        # re_serialized = bundle.build()
        # # Finally, confirm the round trip.
        # self.assertEqual(
        #     re_serialized,
        #     _helpers.deserialize_bundle(re_serialized, client=client).build(),
        # )