Groupby and transform on DataFrame produces FutureWarning
Using pandas 0.10.1:

```
df1 = pd.DataFrame(np.random.randn(10))
df1.groupby(np.arange(len(df1))).transform(lambda x: x)
```

produces:

```
FutureWarning: rename with inplace=True  will return None from pandas 0.11 onward.
```

In the release notes for 0.10.1 I saw the changes regarding functions with the `inplace` option. However, it didn't seem like the above code should naturally lead to such a warning. No warning is produced if the same operation is done on a Series.
