group.apply(...) changes dtype to object -> Regression?
``` python
import pandas
df = pandas.DataFrame({"a":["a","b","a","b"], "b":[1,2,3,4], "c":[1.,2.,3.,4.], "d":["a","b","a","b"]})
print df.dtypes
print df
print df[["b","c"]].corr()
groups = df.groupby("a")
highest = groups.apply(lambda group: group.ix[group['b'].idxmax()])
print highest.dtypes
print highest
print highest[["b","c"]].corr()
```

I expected that the dtypes were not changed, e.g. columns b and c should stay int and float.

I had that in my notebook and reran it with a newer version (about 2 month ago vs compiled today) and was wondering why some columns didn't show up in the describe and corr calls...
