BUG: csv w only header and no index_col failing
after this fix https://github.com/pydata/pandas/pull/10302

The following is failing (as `index_col` is `None`)

```
In [9]: data
Out[9]: 'header1,header2\n'

In [10]: read_csv(StringIO(data),iterator=True,index_col=False).read()      
TypeError: object of type 'bool' has no len()
```
