fix crash when allow_cellular is null on iOS (#774)

Author: rupinderjeet

lib/src/downloader.dart

@@ -162,7 +162,9 @@ class FlutterDownloader {
             filename: item['file_name'] as String?,
             savedDir: item['saved_dir'] as String,
             timeCreated: item['time_created'] as int,
-            allowCellular: item['allow_cellular'] as bool,
+
+            // allowCellular field is true by default (similar to enqueue())
+            allowCellular: (item['allow_cellular'] as bool?) ?? true,
           );
         },
       ).toList();