`FileStore` backend assumes keys are strings

[Riccardo Murri]

The file:// backing store assumes keys are strings and passes them directly to urllib.quote_plus. The following code demoes the issue::

    #! /usr/bin/env python
    #

    from tempfile import mkdtemp

    import shove

    # make temp dir
    tmpdir = mkdtemp(prefix='issue3.', suffix='.tmp.d')

    store_uri = 'file://' + tmpdir

    data = shove.Shove(store=store_uri)

    for n in range(10):
        data[n] = str(n*n)

Indeed::

     $ ./shove_issue_3.py
     Traceback (most recent call last):
       File "./gc3libs/shove_issue_3.py", line 16, in <module>
         data[n] = str(n*n)
       File ".../shove/core.py", line 44, in __setitem__
         self.sync()
       File ".../shove/core.py", line 74, in sync
         self._store.update(self._buffer)
       File ".../lib/python2.7/_abcoll.py", line 566, in update
         self[key] = other[key]
       File ".../shove/base.py", line 120, in __setitem__
         with open(self._key_to_file(key), 'wb') as item:
       File ".../shove/base.py", line 154, in _key_to_file
         return join(self._dir, quote_plus(key))
       File "/usr/lib/python2.7/urllib.py", line 1303, in quote_plus
         if ' ' in s:
     TypeError: argument of type 'int' is not iterable