add example for get and set http header

examples/header.rs

@@ -0,0 +1,27 @@
+use async_std::task;
+use tide::{Request, Response, StatusCode};
+
+fn main() -> Result<(), std::io::Error> {
+    task::block_on(async {
+        let mut app = tide::new();
+
+        app.at("/").get(|req: Request<()>| async move {
+            let content_type = req
+                .header(&"accept".parse().unwrap())
+                .unwrap()
+                .get(0)
+                .unwrap()
+                .as_str();
+
+            let res = Response::new(StatusCode::Ok)
+                .body_string(content_type.to_owned())
+                .set_header("content-type".parse().unwrap(), "text/html");
+
+            Ok(res)
+        });
+
+        app.listen("127.0.0.1:8080").await?;
+
+        Ok(())
+    })
+}