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/cache_control.rs

@@ -10,7 +10,7 @@ pub struct CacheControl(pub Vec<CacheDirective>);
 deref!(CacheControl => Vec<CacheDirective>);
 
 impl Header for CacheControl {
-    fn header_name(_: Option<CacheControl>) -> &'static str {
+    fn header_name() -> &'static str {
         "Cache-Control"
     }