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/lib.rs

@@ -24,6 +24,7 @@ extern crate futures_cpupool;
 #[cfg(feature = "compat")]
 extern crate http;
 extern crate httparse;
+extern crate iovec;
 extern crate language_tags;
 #[macro_use] extern crate log;
 pub extern crate mime;