refactor(lib): moved LanguageTag export to header

BREAKING CHANGE: LanguageTag used to be at the crate root, but it is now
  in the `hyper::header` module.

src/header/shared/mod.rs

@@ -4,6 +4,7 @@ pub use cookie::CookieJar;
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
 pub use self::httpdate::HttpDate;
+pub use language_tags::LanguageTag;
 pub use self::quality_item::{Quality, QualityItem, qitem, q};
 
 mod charset;