There seems to be a difference between .agg and .groupby.agg
This
`pd.DataFrame([[1,2,1],[3,4,1]], columns=['a', 'b', 'dummygroup']).groupby('dummygroup').agg({'a': [('spread', lambda x: x.max()-x.min())]})`
works.

This
`pd.DataFrame([[1,2,1],[3,4,1]], columns=['a', 'b', 'dummygroup']).agg({'a': [('spread', lambda x: x.max()-x.min())]})`
raises an error:

> AttributeError: 'spread' is not a valid function for 'Series' object

Only difference is leaving out the dummy grouping.

Using newest pandas version '1.2.1'
