Column information lost in to_json() if data frame is empty
Create empty dataframe with a column and write it out with to_json(), then read it back in:

```
In [55]: s = pandas.DataFrame({'test': [] }, index=[] ).to_json(orient='columns')
In [56]: s
Out[56]: '{}'
In [57]: str( pandas.read_json(s, orient='columns') )
Out[57]: 'Empty DataFrame\nColumns: []\nIndex: []'
```

I think the expected string is '{"test":{}}'

This situation arises when code filters data frames and writes them out, while not being very careful to make sure the filtered frames contain at least one row.
