test(headers): Add tests for possibly empty list headers.

src/header/common/access_control_allow_headers.rs

@@ -8,4 +8,6 @@ header! {
     #[doc="response to a preflight request, which header field names can be used"]
     #[doc="during the actual request."]
     (AccessControlAllowHeaders, "Access-Control-Allow-Headers") => (UniCase<String>)*
-}
+    
+    test_access_control_allow_headers {}
+}