FInstancedStruct does not serialise to JSON when used in a DataTable

The JSON export functionality for DataTable relies on a FieldIterator to gather properties. This iterator is unable to gather the properties from the InstancedStruct.

CSV export functionality works as it does not rely on FieldIterator.

1. Create a struct with any BP editable members
2. Create a struct that inherits from DataTableRow and has an FInstanceStruct member that uses the first struct type
3. Create a data table in the editor that uses this second struct as a row type
4. Fill a row and save the table
5. Attempt to export the table as JSON

Expected: The table with export the table faithfully as in JSON
Actual: InstancedStructs are not exported.

UScriptStruct::ExportText
Class.cpp:3482