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/expires.rs

@@ -12,7 +12,7 @@ pub struct Expires(pub Tm);
 deref!(Expires => Tm);
 
 impl Header for Expires {
-    fn header_name(_: Option<Expires>) -> &'static str {
+    fn header_name() -> &'static str {
         "Expires"
     }