Data frame (#50)

* Rename DataFlag -> DataFlags
* Polish Data frame API

tests/support/src/codec.rs

@@ -5,6 +5,16 @@ macro_rules! assert_closed {
     }}
 }
 
+#[macro_export]
+macro_rules! poll_err {
+    ($transport:expr) => {{
+        match $transport.poll() {
+            Err(e) => e,
+            frame => panic!("expected error; actual={:?}", frame),
+        }
+    }}
+}
+
 #[macro_export]
 macro_rules! poll_data {
     ($transport:expr) => {{