GridModel should still allow custom pinned rows

[ghsolomon]

Developers should be able to utilize AG-Grid's support for custom pinned rows, but currently GridModel overwrites pinned row data provided via agOptions. As a workaround, pinned rows can be set by calling agApi.setPinnedTopRowData(), but currently the data provided must be StoreRecords, which is not ideal because we don't expect application code to construct StoreRecords. Consider changing Hoist methods that deal with pinned row data to first confirm that the data is instanceof StoreRecord.