Cannot use system property as a page provider aggregate

1. in Studio, create a page provider using a system property like ecm:primaryType (see ppConfig.png)
2. configure the search to use it (see https://doc.nuxeo.com/nxdoc/web-ui-search/ for detailed steps)
3. when clicking the search button the following error appears in the log:

   2018-08-29 10:50:40,715 WARN  [http-nio-0.0.0.0-8080-exec-14] [org.nuxeo.elasticsearch.io.marshallers.json.AggregateJsonWriter] Could not resolve field ecm:primaryType for aggregate system_primaryType_agg
   

4. the issue seems to lie here:
   https://github.com/nuxeo/nuxeo/blob/master/nuxeo-features/nuxeo-elasticsearch/nuxeo-elasticsearch-core/src/main/java/org/nuxeo/elasticsearch/io/marshallers/json/AggregateJsonWriter.java#L103
5. since the system property is not part of the schema, it is not found

Expected behavior: no error should be displayed and the search performed correctly. This would allow for instance customers to use document types in aggregations.