Add special handling of set-cookie to Headers

fetch.bs

@@ -732,12 +732,30 @@ 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 `<code>set-cookie</code>`, then:
+
+    <ol>
+     <li><p>Let <var>values</var> be a list of all <a lt=value for=header>values</a> of
+      <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.
+
+     <li><p><a for=list>For each</a> <var>value</var> of <var>values</var>:
+
+      <ol>
+       <li><p><a for=list>Append</a> (<var>name</var>, <var>value</var>) to <var>headers</var>.
+      </ol>
+    </ol>
+
+   <li><p>Otherwise:
+
+    <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>
   </ol>
 
  <li><p>Return <var>headers</var>.
@@ -5960,18 +5978,25 @@ interface Headers {
   undefined append(ByteString name, ByteString value);
   undefined delete(ByteString name);
   ByteString? get(ByteString name);
+  sequence&lt;ByteString> getSetCookie();
   boolean has(ByteString name);
   undefined set(ByteString name, ByteString value);
   iterable&lt;ByteString, ByteString>;
 };
 </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 cannot be safely combined. Because
+of this, there is some special handling for the `<code>Set-Cookie</code>` header in
+the implementation of the {{Headers}} interface, just like in the HTTP spec.
+`<code>Set-Cookie</code>` headers are not concatenated when they are returned from the {{Headers}}
+iterator, and there is a
+<code><var>headers</var> . <a method for=Headers lt=getSetCookie()>getSetCookie</a>()</code> method
+to retrieve the list of all `<code>Set-Cookie</code>`, ordered and uncombined. This means that
+specifically for this header, multiple tuples of the same name can be returned from the {{Headers}}
+iterator. 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
@@ -6018,6 +6043,9 @@ new Headers(meta2);
  <dd><p>Returns as a string the values of all headers whose name is <var>name</var>, separated by a
  comma and a space.
 
+ <dt><code><var>headers</var> . <a method for=Headers lt=getSetCookie()>getSetCookie</a>()</code>
+ <dd><p>Returns a list of the values for all headers whose name is `<code>Set-Cookie</code>`.
+
  <dt><code><var>headers</var> . <a method for=Headers lt=has()>has</a>(<var>name</var>)</code>
  <dd><p>Returns whether there is a header whose name is <var>name</var>.
 
@@ -6160,6 +6188,17 @@ method steps are to <a for=Headers>append</a> (<var>name</var>, <var>value</var>
  <a for=Headers>header list</a>.
 </ol>
 
+<p>The <dfn export for=Headers method><code>getSetCookie()</code></dfn> method steps are:
+
+<ol>
+ <li><p>If <var>list</var> <a for="header list">does not contain</a> `<code>Set-Cookie</code>`, then
+ return an empty list.
+
+ <li><p>Return the <a lt=value for=header>values</a> of all <a for=/>headers</a> in <a>this</a>'s
+ <a for=Headers>header list</a> whose <a for=header>name</a> is a <a>byte-case-insensitive</a> match
+ for `<code>Set-Cookie</code>`, in order.
+</ol>
+
 <p>The <dfn export for=Headers method><code>has(<var>name</var>)</code></dfn> method steps are:
 
 <ol>