refactor(headers): factor out EntityTag from Etag

Allow use of EntityTag in other headers that use entity tags.

BREAKING CHANGE: for any consumers of the Etag header, since the entity
tag is now in a tuple.

src/header/mod.rs

@@ -22,7 +22,7 @@ use unicase::UniCase;
 
 use {http, HttpResult, HttpError};
 
-pub use self::shared::{Encoding, QualityItem, qitem};
+pub use self::shared::{Encoding, EntityTag, QualityItem, qitem};
 pub use self::common::*;
 
 mod common;