df.apply has problems storing tuples into new column
I am quite perplexed about this: 
A function applied to rows of dataframes that returns a tuple throws an error if the result is stored into a new column, but does not if the column already exists? This does not happen if I return just a number.
Get the example notebook here:
https://gist.github.com/a83ed19c43fe2eaac6a4
or see it here:
https://nbviewer.jupyter.org/gist/michaelaye/a83ed19c43fe2eaac6a4

Using master pandas. ('0.16.2+442.g24c147a', 24c147a369d4)
