Adding receipts (#20)

spec/fixtures/refund_request.json

@@ -0,0 +1,8 @@
+{
+  "amount": {
+    "value": "8.00",
+    "currency": "RUB"
+  },
+  "payment_id": "40484c25-1095-4d36-8b17-92672d1e127d",
+  "description": "test refund, idem-key 40484c25-1095-4d36-8b17-92672d1e127d"
+}