csv writer engine 'fast' has a bug
``` python
rdr2.to_csv('jdate_test_out.csv', index=False)
csvtest = pd.read_csv('jdate_test_out.csv')
print csvtest.jdate.ptp()
rdr2.to_csv('jdate_test_out.csv', index=False, engine='fast')
csvtest = pd.read_csv('jdate_test_out.csv')
print csvtest.jdate.ptp()
```

0.0416644699872
0.0

Disclaimer: the 'fast' engine came from my own PR, so I messed up somewhere.
