* Add `Headers`
Example usage:
```rust
use axum::{
route,
routing::RoutingDsl,
response::{IntoResponse, Headers},
handler::get,
};
use http::header::{HeaderName, HeaderValue};
// It works with any `IntoIterator<Item = (Key, Value)>` where `Key` can be
// turned into a `HeaderName` and `Value` can be turned into a `HeaderValue`
//
// Such as `Vec<(HeaderName, HeaderValue)>`
async fn just_headers() -> impl IntoResponse {
Headers(vec![
(HeaderName::from_static("X-Foo"), HeaderValue::from_static("foo")),
])
}
// Or `[(&str, &str)]`
async fn from_strings() -> impl IntoResponse {
Headers([("X-Foo", "foo")])
}
```
Fixes https://github.com/tokio-rs/axum/issues/187
* Make work on Rust versions without `IntoIterator` for arrays
* format
* changelog
