Theme: fix autocompletion with the POST method

With the POST method, autocomplete.js does not URL encode the values.
For example "1+1" is sent as "1+1" which is read as "1 1" since space are URL encoded with a plus.

There is no clean way to fix the bug since autocomplete.js seems abandoned.

The commit monkey patches the ajax function of autocomplete.js

Related to #1695