AssertionError with df.resample(how="median")
I've reproduced _something_ using `how="median"`, perhaps #1648. It seems to hit when there are discontinuities in the resampling (i.e. minutes with no records when downsampling).

Both pandas 0.8.1 and 0.8.2.dev-f5a74d4 don't like it:

``` python
import pandas as pd
from datetime import datetime
df = pd.DataFrame([1, 2], index=[datetime(2012,1,1,0,0,0), datetime(2012,1,1,0,5,0)])
df.resample("T", how="median")
# Throws AssertionError
```
