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/set_cookie.rs

@@ -16,7 +16,7 @@ pub struct SetCookie(pub Vec<Cookie>);
 deref!(SetCookie => Vec<Cookie>);
 
 impl Header for SetCookie {
-    fn header_name(_: Option<SetCookie>) -> &'static str {
+    fn header_name() -> &'static str {
         "Set-Cookie"
     }