Aggregate sometimes blank

When using the row and column aggregate options, the aggregates are sometimes rendered incorrectly resulting in no values being shown.

For example, I have the pivot values as a sum of the raw data and then have specified 'sum of values' for both rows and columns to calculate totals. In one view, this works correctly but in another (using a different field) the totals are blank.

The total is being calculated correctly but the $aggregate object passed to $field_handler->render does not have the calculated value assigned to the right property.

The attached patch fixes the issue.