add vary header, first draft

src/header/common/mod.rs

@@ -24,6 +24,7 @@ pub use self::location::Location;
 pub use self::transfer_encoding::TransferEncoding;
 pub use self::upgrade::Upgrade;
 pub use self::user_agent::UserAgent;
+pub use self::vary::Vary;
 pub use self::server::Server;
 pub use self::set_cookie::SetCookie;
 
@@ -132,4 +133,7 @@ pub mod upgrade;
 /// Exposes the UserAgent header.
 pub mod user_agent;
 
+/// Exposes the Vary header.
+pub mod vary;
+
 pub mod util;