Show deprecation warnings for header values in XHR according to RFC 7230

LayoutTests/http/tests/fetch/chromium/error-messages.html

Index: LayoutTests/http/tests/fetch/chromium/error-messages.html
diff --git a/LayoutTests/http/tests/fetch/chromium/error-messages.html b/LayoutTests/http/tests/fetch/chromium/error-messages.html
index c153cef2cde43793c06e2c72aef290b592fb086f..970039c439cdb518372f2b8973b10148682f023c 100644
--- a/LayoutTests/http/tests/fetch/chromium/error-messages.html
+++ b/LayoutTests/http/tests/fetch/chromium/error-messages.html
@@ -42,4 +42,13 @@ promise_test(function(t) {
     return fetch('http://localhost:8000/fetch/resources/redirect-loop.php?Count=100&ACAOrigin=*')
       .then(t.unreached_func('must reject'), function() {});
   }, 'A didFailRedirectCheck() error should output an error message');
+
+promise_test(function(t) {	
+    return Promise.resolve()	
+      .then(function() {	
+         var headers = new Headers();	
+         headers.set('a', 't');
+         headers.append('X-Test', 't ');	
+       });	
+  }, 'A deprecation warning should be output for heders.append and set');
 </script>