don't use strings for paths

tests/simple_integration_test.rs

@@ -1,4 +1,5 @@
 use herodb::{server::Server, options::DBOption};
+use std::path::PathBuf;
 use std::time::Duration;
 use tokio::time::sleep;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
@@ -19,7 +20,7 @@ async fn start_test_server(test_name: &str) -> (Server, u16) {
     std::fs::create_dir_all(&test_dir).unwrap();
     
     let option = DBOption {
-        dir: test_dir,
+        dir: PathBuf::from(test_dir),
         port,
         debug: true,
         encrypt: false,