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/upgrade.rs

@@ -39,7 +39,7 @@ impl fmt::String for Protocol {
 }
 
 impl Header for Upgrade {
-    fn header_name(_: Option<Upgrade>) -> &'static str {
+    fn header_name() -> &'static str {
         "Upgrade"
     }