Add Headers response (#193)

* 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

src/response/mod.rs

@@ -13,15 +13,20 @@ use http_body::{
 use std::{borrow::Cow, convert::Infallible};
 use tower::{util::Either, BoxError};
 
+mod headers;
+mod redirect;
+
+pub mod sse;
+
 #[doc(no_inline)]
 pub use crate::Json;
 
-mod redirect;
-
-pub use self::redirect::Redirect;
-
-pub mod sse;
-pub use sse::{sse, Sse};
+#[doc(inline)]
+pub use self::{
+    headers::Headers,
+    redirect::Redirect,
+    sse::{sse, Sse},
+};
 
 /// Trait for generating responses.
 ///