Use the request's current URL instead

Per wanderview's comment. On a null request, I just fall back to the
response's URL. I still haven't located or written web-platform-tests
for this.

source

@@ -82415,8 +82415,13 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
 
     <p><dfn data-x="set the document's address">Setting the document's address</dfn>: Any
     <code>Document</code> created by these steps must have its <span
-    data-x="concept-document-url">URL</span> set to <var>response</var>'s <span
-    data-x="concept-response-url">URL</span>.</p>
+    data-x="concept-document-url">URL</span> set to:</p>
+    <dl class="switch">
+     <dt>If <var>request</var> is non-null,</dt>
+     <dd><var>request</var>'s <span data-x="concept-request-current-url">current URL</span></dd>
+     <dt>Otherwise (<var>request</var> is null),</dt>
+     <dd><var>response</var>'s <span data-x="concept-response-url">URL</span></dd>
+    </dl>
 
     <p><!--en-GB--><dfn id="initialise-the-document-object" data-x="initialize the Document object"
     data-export="">Initializing a new <code>Document</code> object</dfn>: when a