What needs to be fixed

In com_search/controller.php, the searched term isn't encoded when the server send redirection to the client.
The standard is to always encode those characters. A double urlencode() could fix it.

Why this should be fixed

This cause number of bugs with special characters, such as ampersand (place everything that follow it into a new URL parameter), and accents cause errors with some security protections, such as HAproxy's default configuration for accept-invalid-http-response.

How would you fix it

As stated before, replacing $post['searchword'] = $searchword; by $post['searchword'] = urlencode(urlencode($searchword)); fix the problem.
I created this issue to reference it in my Pull Request with the fix.

Side Effects expected

None, except some bugs or unexpected behavior to be gone ?