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.

benches/client.rs

@@ -34,7 +34,7 @@ fn handle(_r: Request, res: Response) {
 struct Foo;
 
 impl hyper::header::Header for Foo {
-    fn header_name(_: Option<Foo>) -> &'static str {
+    fn header_name() -> &'static str {
         "x-foo"
     }
     fn parse_header(_: &[Vec<u8>]) -> Option<Foo> {

benches/client_mock_tcp.rs

@@ -50,7 +50,7 @@ impl Writer for MockStream {
 struct Foo;
 
 impl hyper::header::Header for Foo {
-    fn header_name(_: Option<Foo>) -> &'static str {
+    fn header_name() -> &'static str {
         "x-foo"
     }
     fn parse_header(_: &[Vec<u8>]) -> Option<Foo> {