fix: cast type to int (#25)

Author: spkjp

src/main/java/org/arkecosystem/crypto/transactions/Transaction.java

@@ -220,7 +220,7 @@ public String toJson() {
         map.put("id", this.id);
         map.put("timestamp", this.timestamp);
         map.put("expiration", this.expiration);
-        map.put("type", this.type);
+        map.put("type", this.type.getValue());
         map.put("amount", this.amount);
         map.put("fee", this.fee);
         map.put("recipientId", this.recipientId);