perf(http): utilize writev when possible

By using `AsyncWrite::write_buf`, we can avoid some copies in some
cases. This especially helps throughput for chunked encoding.

src/proto/h1/mod.rs

@@ -1,5 +1,5 @@
 pub use self::decode::Decoder;
-pub use self::encode::Encoder;
+pub use self::encode::{EncodedBuf, Encoder};
 
 mod date;
 mod decode;