don't use strings for paths

tests/debug_hset.rs

@@ -1,4 +1,5 @@
 use herodb::{server::Server, options::DBOption};
+use std::path::PathBuf;
 use std::time::Duration;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::TcpStream;
@@ -22,7 +23,7 @@ async fn debug_hset_simple() {
     
     let port = 16500;
     let option = DBOption {
-        dir: test_dir.to_string(),
+        dir: PathBuf::from(test_dir),
         port,
         debug: false,
         encrypt: false,