Fix #352: Change .tFoot and .createTFoot() to always insert at the end

Since the content model no longer allows `tfoot` before `tbody`,
change these APIs to insert at the end. Firefox already does so,
which makes this seem reasonably safe to change.

source

@@ -37758,18 +37758,14 @@ dictionary <dfn>TrackEventInit</dfn> : <span>EventInit</span> {
   getting, the first <code>tfoot</code> element child of the <code>table</code> element, if any, or
   null otherwise. On setting, if the new value is null or a <code>tfoot</code> element, the first
   <code>tfoot</code> element child of the <code>table</code> element, if any, must be removed, and
-  the new value, if not null, must be inserted immediately before the first element in the
-  <code>table</code> element that is neither a <code>caption</code> element, a <code>colgroup</code>
-  element, nor a <code>thead</code> element, if any, or at the end of the table if there are no such
-  elements. If the new value is neither null nor a <code>tfoot</code> element, then a
-  <code>HierarchyRequestError</code> DOM exception must be thrown instead.</p>
+  the new value, if not null, must be inserted at the end of the table. If the new value is neither
+  null nor a <code>tfoot</code> element, then a <code>HierarchyRequestError</code> DOM exception
+  must be thrown instead.</p>
 
   <p>The <dfn><code data-x="dom-table-createTFoot">createTFoot()</code></dfn> method must return the
   first <code>tfoot</code> element child of the <code>table</code> element, if any; otherwise a new
-  <code>tfoot</code> element must be created and inserted immediately before the first element in
-  the <code>table</code> element that is neither a <code>caption</code> element, a
-  <code>colgroup</code> element, nor a <code>thead</code> element, if any, or at the end of the
-  table if there are no such elements, and then that new element must be returned.</p>
+  <code>tfoot</code> element must be created and inserted at the end of the table, and then that new
+  element must be returned.</p>
 
   <p>The <dfn><code data-x="dom-table-deleteTFoot">deleteTFoot()</code></dfn> method must remove the
   first <code>tfoot</code> element child of the <code>table</code> element, if any.</p>