feat(header): add prefer and preference applied headers

Closes #747

src/header/common/mod.rs

@@ -44,6 +44,8 @@ pub use self::if_range::IfRange;
 pub use self::last_modified::LastModified;
 pub use self::location::Location;
 pub use self::pragma::Pragma;
+pub use self::prefer::{Prefer, Preference};
+pub use self::preference_applied::PreferenceApplied;
 pub use self::range::{Range, ByteRangeSpec};
 pub use self::referer::Referer;
 pub use self::server::Server;
@@ -392,6 +394,8 @@ mod if_unmodified_since;
 mod last_modified;
 mod location;
 mod pragma;
+mod prefer;
+mod preference_applied;
 mod range;
 mod referer;
 mod server;