Broadcasting bug with datetime.date index?
Guys,

It seems to be a problem with broadcasting when having a datetime.date index, do you know what may be causing this?

Example:

``` python
index = index=[datetime.date(2012, 1, 1), datetime.date(2012, 1, 2)]
columns=['a', 'b']
df = pandas.DataFrame([[1., 2.], [3., 4.]], index, columns)

>>> print( df.mean(1) )
2012-01-01    1.5
2012-01-02    3.5

>>> print( (df - df.mean(1)).to_string() )
             a   b  2012-01-01  2012-01-02
2012-01-01 NaN NaN         NaN         NaN
2012-01-02 NaN NaN         NaN         NaN
```

The columns somehow end up being the union of the inital columns and the index. The above code works OK when using datetime.datetime instead.

Many thanks,
Marius
