Add special handling of set-cookie to Headers

fetch.bs

@@ -732,12 +732,34 @@ a <a for=/>header list</a> <var>list</var>, run these steps:
   <p><a for=list>For each</a> <var>name</var> in <var>names</var>:
 
   <ol>
-   <li><p>Let <var>value</var> be the result of <a for="header list">getting</a> <var>name</var>
-   from <var>list</var>.
+   <li>
+    <p>If <var>name</var> is equal to `<code>set-cookie</code>`, then:</p>
+
+    <ol>
+     <li><p>
+      Let <var>values</var> be a list of all <a for=/>headers</a> in <var>list</var> whose
+      <a for=header>name</a> is a <a>byte-case-insensitive</a> match for <var>name</var>, in order.
+     <p></li>
+     <li>
+      <p><a for=list>For each</a> <var>value</var> of <var>values</var>:<p></li>
+
+      <ol>
+       <li><p><a for=list>Append</a> (<var>name</var>, <var>value</var>) to <var>headers</var>.
+      </ol>
+    </ol>
+   </li>
+   <li>
+    <p>Else:</p>
+
+    <ol>
+      <li><p>Let <var>value</var> be the result of <a for="header list">getting</a> <var>name</var>
+      from <var>list</var>.
 
-   <li><p>Assert: <var>value</var> is not null.
+      <li><p>Assert: <var>value</var> is not null.
 
-   <li><p><a for=list>Append</a> (<var>name</var>, <var>value</var>) to <var>headers</var>.
+      <li><p><a for=list>Append</a> (<var>name</var>, <var>value</var>) to <var>headers</var>.
+    </ol>
+   </li>
   </ol>
 
  <li><p>Return <var>headers</var>.
@@ -5966,12 +5988,17 @@ interface Headers {
 };
 </pre>
 
-<p class=note>Unlike a <a for=/>header list</a>, a {{Headers}} object cannot represent more than one
-`<code>Set-Cookie</code>` <a for=/>header</a>. In a way this is problematic as unlike all other
-headers `<code>Set-Cookie</code>` headers cannot be combined, but since `<code>Set-Cookie</code>`
-headers are not exposed to client-side JavaScript this is deemed an acceptable compromise.
-Implementations could choose the more efficient {{Headers}} object representation even for a
-<a for=/>header list</a>, as long as they also support an associated data structure for
+<p class=note>Unlike a <a for=/>header list</a>, a {{Headers}} object combines the values of
+multiple headers of the same name into a single header value. This is problematic for the
+`<code>Set-Cookie</code>` header, as unlike all other headers it can not be safely combined. For
+client-side JavaScript this is not very problematic, as the platform never exposes
+`<code>Set-Cookie</code>`. This can be problematic for server-side runtimes that do expose this
+header. Because of this there is some special handling for the `<code>Set-Cookie</code>` header in
+the implementation of the <a for=headers>`<code>Headers</code>`</a> interface:
+`<code>Set-Cookie</code>` headers are not concatenated when they are returned from the {{Headers}}
+iterator. This means that specifically for this header, multiple tuples of the same name may be
+returned. Implementations could choose the more efficient {{Headers}} object representation even
+for a <a for=/>header list</a>, as long as they also support an associated data structure for
 `<code>Set-Cookie</code>` headers.
 
 <p>A {{Headers}} object has an associated