Wire in trailers (#34)

Add send and receive trailer support.

src/frame/headers.rs

@@ -98,6 +98,7 @@ const ALL: u8 = END_STREAM
 // ===== impl Headers =====
 
 impl Headers {
+    /// Create a new HEADERS frame
     pub fn new(stream_id: StreamId, pseudo: Pseudo, fields: HeaderMap) -> Self {
         Headers {
             stream_id: stream_id,
@@ -108,6 +109,19 @@ impl Headers {
         }
     }
 
+    pub fn trailers(stream_id: StreamId, fields: HeaderMap) -> Self {
+        let mut flags = HeadersFlag::default();
+        flags.set_end_stream();
+
+        Headers {
+            stream_id,
+            stream_dep: None,
+            fields: fields,
+            pseudo: Pseudo::default(),
+            flags: flags,
+        }
+    }
+
     /// Loads the header frame but doesn't actually do HPACK decoding.
     ///
     /// HPACK decoding is done in the `load_hpack` step.
@@ -290,6 +304,10 @@ impl Headers {
         Ok(request)
     }
 
+    pub fn into_fields(self) -> HeaderMap {
+        self.fields
+    }
+
     pub fn encode(self, encoder: &mut hpack::Encoder, dst: &mut BytesMut)
         -> Option<Continuation>
     {