Apply prefer-const lint rule

lib/auth.js

@@ -86,7 +86,7 @@ function digest(username, password, uri, httpMethod, body, authHeader) {
   if (qop === "auth-int") ha2.update(":").update(body);
   ha2 = ha2.digest("hex");
 
-  let hash = crypto.createHash("md5");
+  const hash = crypto.createHash("md5");
   hash
     .update(ha1)
     .update(":")