Support for search api views regionbound cluster aggregation

I tried to get the regionbound clustering working with Search Api, but it is only working with a normal view.

Steps to reproduce:

1. Create a content type with a numeric field (I just made a field with the content defaulting to 1)
2. Set up search api to index this field
3. Create a view of type Search Index
4. Set the "Field to perform aggregation on" under Cluster aggregation to your numeric field
5. Set Aggregation to "Sum"
6. If you display the numeric field on the map, you clearly see a 1 for every result you get, so the values are there to aggregate.

But instead of getting the fields to aggregate, all I see is 0 as the result of the aggregation ("aggregationValue":0, in the source code, so it clearly doesn't compute). If I setup a normal view, I get the correct aggregation results, so it is clear that the aggregation somehow doesn't work with Search Api.

Would be great if search api could be supported as well!