feat(headers): add origin header

Add an Origin header so users may properly send CORS requests

Closes #651

src/header/common/mod.rs

@@ -43,6 +43,7 @@ pub use self::if_unmodified_since::IfUnmodifiedSince;
 pub use self::if_range::IfRange;
 pub use self::last_modified::LastModified;
 pub use self::location::Location;
+pub use self::origin::Origin;
 pub use self::pragma::Pragma;
 pub use self::prefer::{Prefer, Preference};
 pub use self::preference_applied::PreferenceApplied;
@@ -402,6 +403,7 @@ mod if_range;
 mod if_unmodified_since;
 mod last_modified;
 mod location;
+mod origin;
 mod pragma;
 mod prefer;
 mod preference_applied;