to_datetime returns NaT for epoch with errors='coerce'

xref #11760 

Parsing unix epoch timestamps give NaT with `errors='coerce'` while they can be parsed correctly without it:

``` python
>>> pd.to_datetime(1420043460, errors='coerce', unit='s')
NaT
>>> pd.to_datetime(1420043460, unit='s')
Timestamp('2014-12-31 16:31:00')
```

pandas: 0.17.1
numpy: 1.10.1
Python 2.7.7
