(core) Fix bug causing "unmarshallable object" error during conversion to ReferenceList.

Summary:
When a value $B.A is a ReferenceList, returning it in an Any column can cause
an "unmarshallable object" error, if the RecordSet happens to be storing
row_ids in the form of a nested RecordList object. This happened consistently
when $B.A started off as another type and got converted to ReferenceList.

A specific manifestation was when a reference column $B uses "A" as a display
column, and this column gets converted from Text to ReferenceList.

Test Plan: Added a test that reproduces the problem.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3089

sandbox/grist/objtypes.py

@@ -190,7 +190,7 @@ def encode_object(value):
     elif isinstance(value, (list, tuple)):
       return ['L'] + [encode_object(item) for item in value]
     elif isinstance(value, records.RecordSet):
-      return ['r', value._table.table_id, value._row_ids]
+      return ['r', value._table.table_id, value.get_encodable_row_ids()]
     elif isinstance(value, RecordSetStub):
       return ['r', value.table_id, value.row_ids]
     elif isinstance(value, dict):

sandbox/grist/records.py

@@ -142,7 +142,7 @@ class RecordSet(object):
     self._table = table
     self._row_ids = row_ids
     self._source_relation = relation or table._identity_relation
-    # If row_ids is itself a RecordSet, default to its _group_by and _sort_by properties.
+    # If row_ids is itself a RecordList, default to its _group_by and _sort_by properties.
     self._group_by = group_by or getattr(row_ids, '_group_by', None)
     self._sort_by = sort_by or getattr(row_ids, '_sort_by', None)
 
@@ -194,6 +194,17 @@ class RecordSet(object):
                                  group_by=self._group_by,
                                  sort_by=self._sort_by)
 
+  def get_encodable_row_ids(self):
+    """
+    Returns stored rowIds as a simple list or tuple type, even if actually stored as RecordList.
+    """
+    # pylint: disable=unidiomatic-typecheck
+    if type(self._row_ids) in (list, tuple):
+      return self._row_ids
+    else:
+      return list(self._row_ids)
+
+
 
 def adjust_record(relation, value):
   """