feat(headers): add last-event-id header

Add a Last-Event-ID header to properly work with Server-Sent Events

Addresses # 723

src/header/common/last-event-id.rs

@@ -0,0 +1,30 @@
+header! {
+    /// `Last-Event-ID` header, defined in
+    /// [RFC3864](https://html.spec.whatwg.org/multipage/references.html#refsRFC3864)
+    ///
+    /// The `Last-Event-ID` header contains information about
+    /// the last event in an http interaction so that it's easier to
+    /// track of event state. This is helpful when working
+    /// with [Server-Sent-Events](http://www.html5rocks.com/en/tutorials/eventsource/basics/). If the connection were to be dropped, for example, it'd
+    /// be useful to let the server know what the last event you
+    /// recieved was.
+    ///
+    /// The spec is a String with the id of the last event, it can be
+    /// an empty string which acts a sort of "reset".
+    ///
+    /// # Example
+    /// ```
+    /// use hyper::header::{Headers, LastEventID};
+    ///
+    /// let mut headers = Headers::new();
+    /// headers.set(LastEventID("1".to_owned()));
+    /// ```
+    (LastEventID, "Last-Event-ID") => [String]
+
+    test_last_event_id {
+        // Initial state
+        test_header!(test1, vec![b""]);
+        // Own testcase
+        test_header!(test2, vec![b"1"], Some(LastEventID("1".to_owned())));
+    }
+}