perf(header): improve on MemSlice usage in headers

src/http/conn.rs

@@ -595,6 +595,21 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_conn_parse_partial() {
+        let good_message = b"GET / HTTP/1.1\r\nHost: foo.bar\r\n\r\n".to_vec();
+        let io = AsyncIo::new_buf(good_message, 10);
+        let mut conn = Conn::<_, ServerTransaction>::new(io, Default::default());
+        assert!(conn.poll().unwrap().is_not_ready());
+        conn.io.io_mut().block_in(50);
+        let async = conn.poll().unwrap();
+        assert!(async.is_ready());
+        match async {
+            Async::Ready(Some(Frame::Message { .. })) => (),
+            f => panic!("frame is not Message: {:?}", f),
+        }
+    }
+
     #[test]
     fn test_conn_closed_read() {
         let io = AsyncIo::new_buf(vec![], 0);