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/if_modified_since.rs

@@ -12,7 +12,7 @@ pub struct IfModifiedSince(pub Tm);
 deref!(IfModifiedSince => Tm);
 
 impl Header for IfModifiedSince {
-    fn header_name(_: Option<IfModifiedSince>) -> &'static str {
+    fn header_name() -> &'static str {
         "If-Modified-Since"
     }