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/access_control/allow_headers.rs

@@ -15,7 +15,7 @@ pub struct AccessControlAllowHeaders(pub Vec<String>);
 
 impl header::Header for AccessControlAllowHeaders {
     #[inline]
-    fn header_name(_: Option<AccessControlAllowHeaders>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Allow-Headers"
     }
 

src/header/common/access_control/allow_methods.rs

@@ -16,7 +16,7 @@ pub struct AccessControlAllowMethods(pub Vec<method::Method>);
 
 impl header::Header for AccessControlAllowMethods {
     #[inline]
-    fn header_name(_: Option<AccessControlAllowMethods>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Allow-Methods"
     }
 

src/header/common/access_control/allow_origin.rs

@@ -23,7 +23,7 @@ pub enum AccessControlAllowOrigin {
 
 impl header::Header for AccessControlAllowOrigin {
     #[inline]
-    fn header_name(_: Option<AccessControlAllowOrigin>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Allow-Origin"
     }
 

src/header/common/access_control/max_age.rs

@@ -14,7 +14,7 @@ pub struct AccessControlMaxAge(pub u32);
 
 impl header::Header for AccessControlMaxAge {
     #[inline]
-    fn header_name(_: Option<AccessControlMaxAge>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Max-Age"
     }
 

src/header/common/access_control/request_headers.rs

@@ -14,7 +14,7 @@ pub struct AccessControlRequestHeaders(pub Vec<String>);
 
 impl header::Header for AccessControlRequestHeaders {
     #[inline]
-    fn header_name(_: Option<AccessControlRequestHeaders>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Request-Headers"
     }
 

src/header/common/access_control/request_method.rs

@@ -15,7 +15,7 @@ pub struct AccessControlRequestMethod(pub Method);
 
 impl header::Header for AccessControlRequestMethod {
     #[inline]
-    fn header_name(_: Option<AccessControlRequestMethod>) -> &'static str {
+    fn header_name() -> &'static str {
         "Access-Control-Request-Method"
     }