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/cookie.rs

@@ -19,7 +19,7 @@ pub struct Cookies(pub Vec<Cookie>);
 deref!(Cookies => Vec<Cookie>);
 
 impl Header for Cookies {
-    fn header_name(_: Option<Cookies>) -> &'static str {
+    fn header_name() -> &'static str {
         "Cookie"
     }