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/user_agent.rs

@@ -11,7 +11,7 @@ pub struct UserAgent(pub String);
 deref!(UserAgent => String);
 
 impl Header for UserAgent {
-    fn header_name(_: Option<UserAgent>) -> &'static str {
+    fn header_name() -> &'static str {
         "User-Agent"
     }