groupby().max() operation removes string columns
While upgrading pandas from 0.7.2 to 0.9.1 we have found that groupby().max() operation now removes non-numeric columns. This broke our code in several places. Workaround is to use  groupby().aggregate(np.max).

Here is an example demonstrating the problem:

aa=DataFrame({'nn':[11,11,22,22],'ii':[1,2,3,4],'ss':4*['mama']})
aa.groupby('nn').max()

output on pandas 0.7.2
   ii  nn    ss
nn  
11  2  11  mama
22  4  22  mama

output on pandas 0.9.1
    ii
nn  
11   2
22   4

As you see, object column 'ss' is dropped in new version !!!
This was very un-intuitive.
