Bugfix resolving filter object to query params

Bugfix resolving filter object to query params. Thanks to this it will be possible to pass advanced filters to a method for listing vouchers.

Voucherify.listVouchers({
  filters: {
   "junction": "and",
   "redemption.redeemed_quantity": {
     "conditions": {
       "$more_than": 0
     }
   },
   "expiration_date": {
     "conditions": {
       "$more_than": 0
     }
   },
 }
}, function callback (response) { })