feat(headers): Implementing content-encoding header

Closes #391

src/header/common/mod.rs

@@ -16,6 +16,7 @@ pub use self::authorization::{Authorization, Scheme, Basic};
 pub use self::cache_control::{CacheControl, CacheDirective};
 pub use self::connection::{Connection, ConnectionOption};
 pub use self::content_length::ContentLength;
+pub use self::content_encoding::ContentEncoding;
 pub use self::content_type::ContentType;
 pub use self::cookie::Cookie;
 pub use self::date::Date;
@@ -157,6 +158,7 @@ mod authorization;
 mod cache_control;
 mod cookie;
 mod connection;
+mod content_encoding;
 mod content_length;
 mod content_type;
 mod date;