Use request's current url as base URL in redirects

As the response's url list is not set at this point using that doesn not work. We could potentially set it earlier, but that brings along other complications that this setup avoid.

This also returns opaque redirect responses earlier, to avoid having them processed twice, which was simply wrong (and would become extra wrong here).

Fixes #631.

fetch.bs

@@ -3073,6 +3073,9 @@ optional <i>CORS flag</i> and <i>CORS-preflight flag</i>, run these steps:
        not "<code>follow</code>" and <var>response</var>'s
        <a for=response>url list</a> has more than one item.
       </ul>
+
+     <li><p>If <var>response</var>'s <a for=response>type</a> is "<code>opaqueredirect</code>", then
+     return <var>response</var>.
     </ol>
   </ol>
 
@@ -3154,10 +3157,9 @@ optional <i>CORS flag</i> and <i>CORS-preflight flag</i>, run these steps:
    <li><p>Let <var>location</var> be the result of <a>extracting header list values</a> given
    `<code>Location</code>` and <var>actualResponse</var>'s <a for=response>header list</a>.
 
-   <li><p>If <var>location</var> is a <a for=header>value</a>, then set
-   <var>location</var> to the result of
-   <a lt="url parser">parsing</a> <var>location</var> with
-   <var>actualResponse</var>'s <a for=response>url</a>.
+   <li><p>If <var>location</var> is a <a for=header>value</a>, then set <var>location</var> to the
+   result of <a lt="url parser">parsing</a> <var>location</var> with <var>request</var>'s
+   <a for=request>current url</a>.
 
    <li><p>Set <var>actualResponse</var>'s
    <a for=response>location URL</a> to <var>location</var>.