refactor(headers): remove marker from header_name method

It is no longer required, as we can use `<H as Header>::header_name()`.

BREAKING CHANGE: Implementations of Header will need to adjust the
    header_name method. It no longer takes any arguments.

src/header/common/authorization.rs

@@ -23,7 +23,7 @@ impl<S: Scheme> DerefMut for Authorization<S> {
 }
 
 impl<S: Scheme> Header for Authorization<S> {
-    fn header_name(_: Option<Authorization<S>>) -> &'static str {
+    fn header_name() -> &'static str {
         "Authorization"
     }