Can't assign NaT to datetime series with iloc
```
>>> s = pd.Series(pd.date_range('2000-1-1', '2000-1-2'))
>>> s
0   2000-01-01 00:00:00
1   2000-01-02 00:00:00
```

This works:

```
>>> s[0] = pd.NaT
>>> s
0                   NaT
1   2000-01-02 00:00:00
```

This works:

```
>>> s.iloc[0] = pd.NaT.value
>>> s
0                   NaT
1   2000-01-02 00:00:00
dtype: datetime64[ns]
```

This doens't:

```
>>> s.iloc[0] = pd.NaT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/indexing.py", line 88, in __setitem__
    self._setitem_with_indexer(indexer, value)
  File "pandas/core/indexing.py", line 185, in _setitem_with_indexer
    values[indexer] = value
ValueError: Invalid date (-1,-1,-1) when converting to NumPy datetime
```
