Add /healthz healthcheck endpoint (#20)

It merely returns "OK" with a 200 status code

src/service/healthz.rs

@@ -0,0 +1,21 @@
+use crate::http::{ok, Request, Result};
+
+pub async fn healthz(_req: Request) -> Result {
+    ok("OK")
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::test::*;
+    use hyper::http::StatusCode;
+
+    #[tokio::test]
+    async fn test_healthz() {
+        let res = request().handle(healthz).await.unwrap();
+
+        assert_eq!(res.status(), StatusCode::OK);
+        let body = res.read_body_utf8().await.unwrap();
+        assert_eq!(body, "OK");
+    }
+}

src/service/mod.rs

@@ -8,6 +8,7 @@ mod cache;
 mod cookies;
 mod delay;
 mod headers;
+mod healthz;
 mod index;
 mod ip;
 mod method;
@@ -161,5 +162,8 @@ pub fn router() -> Router {
     let routes = std::iter::once(&index_route).chain(builder.routes());
     let index: crate::service::index::Index = routes.into();
 
-    builder.install(index, index_route).build()
+    builder
+        .install(crate::service::healthz::healthz, route(path!("healthz")))
+        .install(index, index_route)
+        .build()
 }