rolling_sum gives ValueError for column name of '64' / DateTimeIndex in year 1964
pandas version: 0.13.1, Python 2.7 32-bit. Windows.

When attempting to calculate rolling sums for a dataframe with a DateTimeIndex in 1964, and naming the column '64' (as a string), I get a ValueError: could not broadcast input array from shape (x) into shape (x, 1). If I change one of the 64s, the error does not occur.

``` python
import pandas as pd

incr = range(150)
datetimes = pd.date_range('1964/01/01 00:00:00', periods=150)

df = pd.DataFrame(data=incr, index=datetimes)
s = pd.Series(data=incr, index=datetimes)
df.columns = ['increments']
df['64'] = pd.rolling_sum(df['increments'], 5)
```

Traceback:

```
Traceback (most recent call last):
  File "C:\Jobs\29075\2014\Software\stormy\pdbug.py", line 9, in <module>
    df['64'] = pd.rolling_sum(df['increments'], 5)
  File "C:\Python27_w32\lib\site-packages\pandas\core\frame.py", line 1879, in __setitem__
    return self._setitem_slice(indexer, value)
  File "C:\Python27_w32\lib\site-packages\pandas\core\frame.py", line 1891, in _setitem_slice
    self.ix._setitem_with_indexer(key, value)
  File "C:\Python27_w32\lib\site-packages\pandas\core\indexing.py", line 422, in _setitem_with_indexer
    self.obj._data = self.obj._data.setitem(indexer, value)
  File "C:\Python27_w32\lib\site-packages\pandas\core\internals.py", line 2396, in setitem
    return self.apply('setitem', *args, **kwargs)
  File "C:\Python27_w32\lib\site-packages\pandas\core\internals.py", line 2376, in apply
    applied = getattr(blk, f)(*args, **kwargs)
  File "C:\Python27_w32\lib\site-packages\pandas\core\internals.py", line 636, in setitem
    values[indexer] = value
ValueError: could not broadcast input array from shape (150) into shape (150,1)
```
