fixed replace in BaseDocument

Author: gschwab

src/main/java/com/arangodb/entity/BaseDocument.java

@@ -182,7 +182,9 @@ public void addAttribute(String key, Object value) {
      * @param value the value of the attribute ti replace the old value
      */
     public void updateAttribute (String key, Object value) {
-        this.properties.replace(key, value);
+        if (this.properties.containsKey(key)) {
+            this.properties.put(key, value);
+        }
     }
 
 //    /**