Resolve while_let_on_iterator clippy lint

    warning: this loop could be written as a `for` loop
        --> serde/src/private/de.rs:2905:9
         |
    2905 |         while let Some(item) = self.iter.next() {
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for item in self.iter.by_ref()`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator
         = note: `#[warn(clippy::while_let_on_iterator)]` on by default

Author: dtolnay

serde/src/private/de.rs

@@ -2902,7 +2902,7 @@ where
     where
         T: DeserializeSeed<'de>,
     {
-        while let Some(item) = self.iter.next() {
+        for item in self.iter.by_ref() {
             if let Some((key, content)) = use_item(item, self.fields) {
                 self.pending_content = Some(content);
                 return seed.deserialize(ContentDeserializer::new(key)).map(Some);