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/content_length.rs

@@ -12,7 +12,7 @@ pub struct ContentLength(pub u64);
 deref!(ContentLength => u64);
 
 impl Header for ContentLength {
-    fn header_name(_: Option<ContentLength>) -> &'static str {
+    fn header_name() -> &'static str {
         "Content-Length"
     }