Timestamp.max truncation when assigning to DataFrame using mask
In pandas 0.13.1 assigning to a DataFrame using a mask causes strange truncation of Timestamp.max values.

Haven't quite managed to get to the bottom of this but here is a standalone repro of the problem.

```
In [1]: from datetime import datetime, timedelta

In [2]: import pandas as pd

In [3]: pd
Out[3]: <module 'pandas' from 'C:\VirtualEnvs\esprit.dev\lib\site-packages\pandas-0.13.1-py2.6-win32.egg\pandas\__init__.pyc'>

In [4]: df = pd.DataFrame(data=[datetime(2013, 1, 2), datetime(2013, 1, 3)], columns=["start_date"])

In [5]: df
Out[5]:
  start_date
0 2013-01-02
1 2013-01-03

[2 rows x 1 columns]

In [6]: pd.Timestamp.max
Out[6]: Timestamp('2262-04-11 23:47:16.854775807', tz=None)

In [7]: df["start_date"][df["start_date"] > datetime(2013, 1, 2)] = pd.Timestamp.max

In [8]: df
Out[8]:
                  start_date
0        2013-01-02 00:00:00
1 2262-04-11 23:47:16.854775

[2 rows x 1 columns]

In [9]: df["start_date"] == pd.Timestamp.max
Out[9]:
0    False
1    False
Name: start_date, dtype: bool

In [10]: df.loc[df["start_date"] > datetime(2013, 1, 2), "start_date"] = pd.Timestamp.max

In [11]: df
Out[11]:
                     start_date
0           2013-01-02 00:00:00
1 2262-04-11 23:47:16.854775807

[2 rows x 1 columns]

In [12]: df["start_date"] == pd.Timestamp.max
Out[12]:
0    False
1     True
Name: start_date, dtype: bool
```

So the masked assignment causes truncation, but if I use `loc` no truncation occurs.

Interestingly, I ran this same code against pandas 0.12 and observed the exact opposite -- using `loc` caused the truncation, but the straight assignment was fine.

What is going on here?!
