DataFrame.groupy renames columns when given a list of tuples - undocumented behavior
#### Code Sample, a copy-pastable example if possible

```python
>>> df = pd.DataFrame({'key' : ['a', 'a', 'b', 'b', 'a'],
                       'data' : np.random.randn(5)})

>>> df.groupby('key')['data'].agg([('foo', 'mean')])
          foo
key          
a   -0.944872
b    0.893212

```
#### Problem description
I know its deprecated to rename columns with a dictionary of dictionaries in a groupby but a list of tuples also renames columns.

This behavior is not found in the docs. I accidentally discovered this a while back and then saw it again in Python for Data Analysis.
