test: add unknown field to mocked response (#217)

This tests the fix for unknown fields provided in
#215

Author: racevedoo

src/test/resources/post_transaction_response.json

@@ -2,6 +2,7 @@
   "id": "dfe1f2ff-8f0d-4ce8-aed1-af8435143044",
   "risk_assessment": "low_risk",
   "device_id": "1df6d999-556d-42c3-8c63-357e5d08d95b",
+  "unknown_field": "unknown_value",
   "reasons": [
     {
       "code": "trusted_location",