Bug#11637 fix to_csv
This is a fix to Bug #11637. 
Error Description : incorrect output when a mix of integer and string column names passed as columns parameter in to_csv(). 
[format.py](https://github.com/pydata/pandas/blob/master/pandas/core/format.py#L1330) is the cause of this discrepancy. 

Please review .

looks good. pls add a whatsnew new for bug fixes for 0.17.1
 ping when green.
 Done .
 merged via https://github.com/pydata/pandas/commit/784445cbe81cc8b2da4e2324689f175cb2a2edbf

thanks!
