refactor(lints): fix unused warnings in tests/docs

examples/client_json.rs

@@ -41,5 +41,6 @@ async fn fetch_json(url: hyper::Uri) -> Result<Vec<User>> {
 #[derive(Deserialize, Debug)]
 struct User {
     id: i32,
+    #[allow(unused)]
     name: String,
 }

src/client/pool.rs

@@ -1005,6 +1005,7 @@ mod tests {
 
     #[derive(Debug)]
     struct CanClose {
+        #[allow(unused)]
         val: i32,
         closed: bool,
     }