Fix order of elements after .add

```js
$('<a>').add('<b>').map((i, e) => console.log(e.tagName))
```

produces `A`, then `B` in jQuery, and `b` and `a` in cheerio.

Adapter from #952.

Co-Authored-By: codeevery <1003657663@qq.com>

lib/api/traversing.js

@@ -410,14 +410,13 @@ exports.end = function() {
 
 exports.add = function(other, context) {
   var selection = this._make(other, context);
-  var contents = uniqueSort(selection.get().concat(this.get()));
+  var contents = uniqueSort(this.get().concat(selection.get()));
 
   for (var i = 0; i < contents.length; ++i) {
-    selection[i] = contents[i];
+    this[i] = contents[i];
   }
-  selection.length = contents.length;
-
-  return selection;
+  this.length = contents.length;
+  return this;
 };
 
 // Add the previous set of elements on the stack to the current set, optionally