Returns a `Hash` object with appropriate fields filled-in.

def GetHashObject(self):
    """Returns a `Hash` object with appropriate fields filled-in."""
    hash_object = rdf_crypto.Hash()
    hash_object.num_bytes = self._bytes_read
    for algorithm in self._hashers:
      setattr(hash_object, algorithm, self._hashers[algorithm].digest())
    return hash_object