feat(headers): add enum for Charset

Make Charset more strongly typed.

src/header/shared/mod.rs

@@ -1,7 +1,9 @@
+pub use self::charset::Charset;
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
 pub use self::quality_item::{Quality, QualityItem, qitem, q};
 
+mod charset;
 mod encoding;
 mod entity;
 mod quality_item;