Response's trailer cannot use [SameObject]

Since an attribute that returns a promise can be made to return a
rejecting promise as well. (We really need [Cached] or some such.)

Tests: web-platform-tests/wpt#4811.

Fixes #473.

fetch.bs

@@ -5044,7 +5044,7 @@ interface Response {
   readonly attribute boolean ok;
   readonly attribute ByteString statusText;
   [SameObject] readonly attribute Headers headers;
-  [SameObject] readonly attribute Promise<Headers> trailer;
+  readonly attribute Promise<Headers> trailer;
 
   [NewObject] Response clone();
 };