Notebooks: fix save as csv/excel/json/xml (#14882)

* add getEncoding method to notebooks

* fix row 501 missing in serialization

* fix row index

* pr comment

src/sql/platform/serialization/common/serializationService.ts

@@ -119,6 +119,10 @@ export class SerializationService implements ISerializationService {
 			let index = 0;
 			let startRequestParams = this.createStartRequest(serializationRequest, index);
 			index = index + startRequestParams.rows.length;
+			// Adjust row index based on whether or not header row is included
+			if (serializationRequest.includeHeaders) {
+				index--;
+			}
 
 			let startResult = await provider.startSerialization(startRequestParams);