BUG: pandas.Series.dt.round inconsistent behaviour on NAT's with different arguments?
#### Code Sample

```python
# Your code here
import pandas as pd
from datetime import datetime
d=pd.DataFrame(data=[[datetime(2010,1,1,23,14,12,599), 2], [None, 4]], columns=['dtime', 'int'])
print d['dtime'].dt.round('s')
print d['dtime'].dt.round('5s')
print d['dtime'].dt.round('min')
```
#### Problem description

This is the output I get:

```python
0   2010-01-01 23:14:12
1                   NaT
Name: dtime, dtype: datetime64[ns]

0   2010-01-01 23:14:10.000000000
1   2262-04-10 00:12:44.999999488
Name: dtime, dtype: datetime64[ns]

0   2010-01-01 23:14:00
1   2262-04-10 00:13:00
Name: dtime, dtype: datetime64[ns]
```

In the first case (freq argument 's') NaT are preserved (I would say, as I expect). However, in the second and third case, NaT's are converted to some apparently weird date time. If I don't miss some particular information (which in case after I googled and browsed the docs shouldn't be that hidden), this seems to be a bug

#### Expected Output
0   2010-01-01 23:14:12
1                   NaT
Name: dtime, dtype: datetime64[ns]

0   2010-01-01 23:14:10.000000000
1                   NaT
Name: dtime, dtype: datetime64[ns]

0   2010-01-01 23:14:00
1                   NaT
Name: dtime, dtype: datetime64[ns]

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.1
nose: None
pip: 9.0.1
setuptools: 24.0.2
Cython: None
numpy: 1.11.1
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: 3.6.0
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
