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/connection.rs

@@ -50,7 +50,7 @@ impl fmt::String for ConnectionOption {
 }
 
 impl Header for Connection {
-    fn header_name(_: Option<Connection>) -> &'static str {
+    fn header_name() -> &'static str {
         "Connection"
     }