error: the borrowed expression implements the required traits
        --> tests/test.rs:1931:34
         |
    1931 |     assert!(serde_json::to_value(&map).is_err());
         |                                  ^^^^ help: change this to: `map`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
         = note: `-D clippy::needless-borrow` implied by `-D clippy::all`

Add clone_from and use IndexMap::extend