Use the default node.js query string library

The `qs` library which was used filtered out stuff like `isVatExempt=1`. In the query it only left `&isVatExempt`, without any value. Cheddargetter didn't understand that a customer was now vatExempt.

The only real change is:

    -var qs = require("qs");
    +var qs = require("querystring");