fix(tree): Fix tree nested data source

Author: tinayuangao

src/material-experimental/tree/data-source/nested-data-source.ts

@@ -28,7 +28,7 @@ export class MatTreeNestedDataSource<T> implements DataSource<T> {
   set data(value: T[]) { this._data.next(value); }
 
   connect(collectionViewer: CollectionViewer): Observable<T[]> {
-    return merge([collectionViewer.viewChange, this._data])
+    return merge(...[collectionViewer.viewChange, this._data])
       .pipe(map(() => {
         return this.data;
       }));