Add test infrastructure to work directly with frames (#56)

This adds a `Codec` based testing API. This is a bit less annoying than writing
at the raw H2 wire protocol level...

src/frame/ping.rs

@@ -12,6 +12,13 @@ pub struct Ping {
 }
 
 impl Ping {
+    pub fn new() -> Ping {
+        Ping {
+            ack: false,
+            payload: Payload::default(),
+        }
+    }
+
     pub fn pong(payload: Payload) -> Ping {
         Ping { ack: true, payload }
     }
@@ -20,6 +27,11 @@ impl Ping {
         self.ack
     }
 
+    #[cfg(feature = "unstable")]
+    pub fn payload(&self) -> &Payload {
+        &self.payload
+    }
+
     pub fn into_payload(self) -> Payload {
         self.payload
     }