MINOR: Relates to #2019, fix broken storageLevelOffset

@here/harp-webtile-datasource/lib/HereWebTileDataSource.ts

@@ -399,10 +399,10 @@ export class HereWebTileDataSource extends WebTileDataSource {
             minDataLevel: 1,
             maxDataLevel: 20,
             resolution: m_options.resolution,
-            dataProvider: new HereTileProvider(m_options)
+            dataProvider: new HereTileProvider(m_options),
+            storageLevelOffset: m_options.storageLevelOffset ?? -1
         });
         this.cacheable = true;
-        this.storageLevelOffset = -1;
         if (this.resolution === WebTileDataSource.resolutionValue.resolution512) {
             this.maxDataLevel = 19; // 512x512 tiles do not have z19
         }

@here/harp-webtile-datasource/test/HereWebTileDataSourceTest.ts

@@ -70,4 +70,23 @@ describe("HereWebTileDataSource", function() {
     it("#createWebTileDataSource throws w/ missing appId", async function() {
         assert.throw(() => new HereWebTileDataSource({ appCode: "42" } as any));
     });
+    it("#createWebTileDataSource check storageLevelOffset", async function() {
+        const apikey = "123";
+        for (let storageLevelOffset = -2; storageLevelOffset <= 2; storageLevelOffset++) {
+            const webTileDataSource = new HereWebTileDataSource({
+                apikey,
+                tileBaseAddress: HereTileProvider.TILE_AERIAL_SATELLITE,
+                storageLevelOffset
+            });
+            assert(webTileDataSource.storageLevelOffset === storageLevelOffset);
+        }
+
+        // Test the default value
+        const webTileDataSource = new HereWebTileDataSource({
+            apikey,
+            tileBaseAddress: HereTileProvider.TILE_AERIAL_SATELLITE,
+            storageLevelOffset: undefined
+        });
+        assert(webTileDataSource.storageLevelOffset === -1);
+    });
 });