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/shared/mod.rs

@@ -1,5 +1,7 @@
 pub use self::encoding::Encoding;
+pub use self::entity::EntityTag;
 pub use self::quality_item::{QualityItem, qitem};
 
 mod encoding;
+mod entity;
 mod quality_item;