Make document's character encoding reflect byte order mark

This change fixes a bug where document's character encoding was set to the return value of the encoding sniffing algorithm rather than to the actual encoding used, which differed when the stream started with a byte order mark. This change incorporates BOM sniffing into the encoding sniffing algorithm, ensuring both encodings are identical.

Tests: web-platform-tests/wpt#22276.

Closes #1077.

source

@@ -2497,6 +2497,9 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
      <li>The <dfn data-x-href="https://encoding.spec.whatwg.org/#utf-8-encode">UTF-8 encode</dfn>
      algorithm which takes a character stream and returns a byte stream</li>
+
+     <li>The <dfn data-x-href="https://encoding.spec.whatwg.org/#bom-sniff">BOM sniff</dfn>
+     algorithm which takes a byte stream and returns an encoding or null.</li>
     </ul>
 
    </dd>
@@ -104983,6 +104986,16 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
 
   <ol>
 
+   <li>
+    <p>If the result of <span data-x="BOM sniff">BOM sniffing</span> is an encoding, return that
+    encoding with <span data-x="concept-encoding-confidence">confidence</span> <i>certain</i>.</p>
+
+    <p class="note">Although the <span>decode</span> algorithm will itself change the encoding to
+    use based on the presence of a byte order mark, this algorithm sniffs the BOM as well in order
+    to set the correct <span>document's character encoding</span> and <span
+    data-x="concept-encoding-confidence">confidence</span>.</p>
+   </li>
+
    <li>
 
     <p>If the user has explicitly instructed the user agent to override the document's character