2nd level dimension store_assemblies policy when querying a cache

[ness31]

When several sub-dimension values are returned from a 2nd level dimension back-end query, the resulting tile should be able to be cached using the <store_assemblies> tag.
However <store_assemblies>set to true, only works when querying a data source; when querying a cache the server returns an error being unable to create the assembly cache (type sqlite3):

sqlite backend failed to open db ... unable to open database file