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/vary.rs

@@ -15,7 +15,7 @@ pub enum Vary {
 }
 
 impl Header for Vary {
-    fn header_name(_: Option<Vary>) -> &'static str {
+    fn header_name() -> &'static str {
         "Vary"
     }