Fix a bug in case the SimpleAggregateFunction type has a comma in its definition.

[palkeo]

SimpleAggregateFunction(something, Tuple(String, String)) would trigger that bug as the inner variable would be Tuple(String.

This fixes it by splitting only on the first comma that it encounters. I assume there is no comma in the function name.

[long2ice]

Thanks! Could you add a unittest?

[palkeo]

Done.