Possible bug instantiating chest with data

[cowlicks]

Maybe I'm not using Chest right

>>> from chest import Chest
>>> c = Chest({'k': 6})
>>> assert 'k' in c
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

[mrocklin]

Yup, looks like in these lines

        self.inmem = data or dict()
        # A set of keys held both in memory or on disk
        self._keys = {}

We need to dump set(data) into self_keys if data is not None