dtype mismatch when calling .median()
Using python 2.5.2, pandas 0.3 and numpy 1.5.1 on a debian lenny box... mean() works, but not median()

```
>>> df1['b'].truncate(before=dt.date(2010,12,2),after=dt.date(2010,12,4))
2010-12-02    6
2010-12-03    7
2010-12-04    8
>>> df1['b'].truncate(before=dt.date(2010,12,2),after=dt.date(2010,12,4)).median()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "build/bdist.linux-i686/egg/pandas/core/series.py", line 486, in median
 File "moments.pyx", line 70, in tseries.median (pandas/lib/src/tseries.c:5967)
 File "moments.pyx", line 28, in tseries.kth_smallest (pandas/lib/src/tseries.c:5539)
ValueError: Buffer dtype mismatch, expected 'double_t' but got 'long'
>>> df1['b'].truncate(before=dt.date(2010,12,2),after=dt.date(2010,12,4)).mean()
7.0
```
