Support writing continuation frames. (#198)

Large header sets might require being split up across multiple frames.
This patch adds support for doing so.

tests/codec_write.rs

@@ -1 +1,86 @@
+#[macro_use]
+pub mod support;
+use support::prelude::*;
 
+#[test]
+fn write_continuation_frames() {
+    // An invalid dependency ID results in a stream level error. The hpack
+    // payload should still be decoded.
+    let _ = ::env_logger::init();
+    let (io, srv) = mock::new();
+
+    let large = build_large_headers();
+
+    // Build the large request frame
+    let frame = large.iter().fold(
+        frames::headers(1).request("GET", "https://http2.akamai.com/"),
+        |frame, &(name, ref value)| frame.field(name, &value[..]));
+
+    let srv = srv.assert_client_handshake()
+        .unwrap()
+        .recv_settings()
+        .recv_frame(frame.eos())
+        .send_frame(
+            frames::headers(1)
+                .response(204)
+                .eos(),
+        )
+        .close();
+
+    let client = Client::handshake(io)
+        .expect("handshake")
+        .and_then(|(mut client, conn)| {
+            let mut request = Request::builder();
+            request.uri("https://http2.akamai.com/");
+
+            for &(name, ref value) in &large {
+                request.header(name, &value[..]);
+            }
+
+            let request = request
+                .body(())
+                .unwrap();
+
+            let req = client
+                .send_request(request, true)
+                .expect("send_request1")
+                .0
+                .then(|res| {
+                    let response = res.unwrap();
+                    assert_eq!(response.status(), StatusCode::NO_CONTENT);
+                    Ok::<_, ()>(())
+                });
+
+            conn.drive(req)
+                .and_then(move |(h2, _)| {
+                    h2.unwrap()
+                })
+        });
+
+    client.join(srv).wait().expect("wait");
+}
+
+fn build_large_headers() -> Vec<(&'static str, String)> {
+    vec![
+        ("one", "hello".to_string()),
+        ("two", build_large_string('2', 4 * 1024)),
+        ("three", "three".to_string()),
+        ("four", build_large_string('4', 4 * 1024)),
+        ("five", "five".to_string()),
+        ("six", build_large_string('6', 4 * 1024)),
+        ("seven", "seven".to_string()),
+        ("eight", build_large_string('8', 4 * 1024)),
+        ("nine", "nine".to_string()),
+        ("ten", build_large_string('0', 4 * 1024)),
+    ]
+}
+
+fn build_large_string(ch: char, len: usize) -> String {
+    let mut ret = String::new();
+
+    for _ in 0..len {
+        ret.push(ch);
+    }
+
+    ret
+}