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/transfer_encoding.rs

@@ -22,7 +22,7 @@ pub struct TransferEncoding(pub Vec<Encoding>);
 deref!(TransferEncoding => Vec<Encoding>);
 
 impl Header for TransferEncoding {
-    fn header_name(_: Option<TransferEncoding>) -> &'static str {
+    fn header_name() -> &'static str {
         "Transfer-Encoding"
     }