Fix data type of created_at field in Fill

Author: Gerschtli

model/src/main/java/com/coinbase/exchange/model/Fill.java

@@ -1,6 +1,7 @@
 package com.coinbase.exchange.model;
 
 import java.math.BigDecimal;
+import java.time.Instant;
 
 /**
  * Created by irufus on 2/18/15.
@@ -10,7 +11,7 @@ public class Fill {
     private String product_id;
     private BigDecimal size;
     private String order_id;
-    private String created_at;
+    private Instant created_at;
     private String liquidity;
     private BigDecimal fee;
     private Boolean settled;
@@ -40,11 +41,11 @@ public void setFee(BigDecimal fee) {
         this.fee = fee;
     }
 
-    public String getCreated_at() {
+    public Instant getCreated_at() {
         return created_at;
     }
 
-    public void setCreated_at(String created_at) {
+    public void setCreated_at(Instant created_at) {
         this.created_at = created_at;
     }