groupby float column with as_index=False will return an object column
I'm using version 0.10.0

```
import pandas as pd
import numpy as np

df = pd.DataFrame({
    "A":np.random.randint(0, 5, 20).astype(float), 
    "B":np.random.random(20)})
df2 = df.groupby("A", as_index=False).mean()
print df2.dtypes
```

the result is

```
A     object
B    float64
```

I think this is because float index use object array, but I think the dtype of the result should be float64. Compare this with apply:

```
df2 = df.groupby("A", as_index=False).apply(lambda x:x.mean())
print df2.dtypes
```

the result is

```
A    float64
B    float64
```
