support async gzip decoding

src/async_impl/mod.rs

@@ -1,11 +1,13 @@
 #![cfg_attr(not(features = "unstable"), allow(unused))]
 
 pub use self::body::{Body, Chunk};
+pub use self::decoder::{Decoder, ReadableChunks};
 pub use self::client::{Client, ClientBuilder};
 pub use self::request::{Request, RequestBuilder};
 pub use self::response::Response;
 
 pub mod body;
 pub mod client;
+pub mod decoder;
 mod request;
 mod response;