[FIX] web: Correctly aggregate values in exported parent groups

When exporting a grouped list view with some nested groups, the aggregate value
of parent groups are not correct. It always sums aggregated values of children
whether the group operator is 'sum' or not (could be 'max', 'avg', ...).

This behavior is wrong and can even lead to a crash if the aggregated field is a
date field (e.g. with group_operator='max'). (Try two sum two dates...)

The quick fix 85cf47fc was merged just before OXP to avoid any crash. This fix
limited the support of aggregates to only int and float fields.
This commit remove this limitation.

This commit correclty implements the aggregation for parent group for all
field types and all group_operator.

This commit also improves the export feature tests.