UMCS-655 Prevent infinite loop caused by Payment <-> AbstractTransaction circular dependency.

Author: Vladislav Lipianin

src/main/java/com/unzer/payment/AbstractTransaction.java

@@ -47,7 +47,7 @@ public abstract class AbstractTransaction<T extends AbstractPayment> implements
     private String type;
     private AdditionalTransactionData additionalTransactionData;
     @JsonFieldIgnore
-    private T payment;
+    private transient T payment;
     @JsonFieldIgnore
     private transient Unzer unzer;
     @JsonFieldIgnore