'Given a flat Python dictionary of keys/values, converts it into the
nested dictionary DynamoDB expects.
Converts::
\'username\': \'john\',
\'tags\': [1, 2, 5],
...to...::
\'username\': {\'S\': \'john\'},
\'tags\': {\'NS\': [\'1\', \'2\', \'5\']},'

def _encode_keys(self, keys):

    raw_key = {}
    for (key, value) in keys.items():
        raw_key[key] = self._dynamizer.encode(value)
    return raw_key
