fix folder type issue

add get folder test

src/SmartsheetClient.php

@@ -91,7 +91,7 @@ public function getRow(string $sheetId, string $rowId): Row
      */
     public function getFolder(string $folderId): Folder
     {
-        return new Folder($this, $this->get("folders/$folderId"));
+        return new Folder($this, (array) $this->get("folders/$folderId"));
     }
 
     /**

tests/SheetsTest.php

@@ -19,6 +19,11 @@ protected function getClient(): SmartsheetClient
         return new SmartsheetClient(['token' => getenv('SMARTSHEET_API_TOKEN')]);
     }
 
+    public function testCanFetchFolder(): void {
+        $folder = $this->getClient()->getFolder('2877722042099588');
+        $this->assertNotNull($folder);
+    }
+
     public function testCanFetchSheets(): void
     {
         $sheetList = $this->getClient()->listSheets();