Use Map instead of HashMap

Author: Caitlin Bales (MSFT)

src/main/java/com/microsoft/graph/serializer/DefaultSerializer.java

@@ -158,7 +158,7 @@ private JsonObject getChildAdditionalData(IJsonBackedObject serializableObject,
 					Iterator<Entry<String, Object>> it = serializableChildren.entrySet().iterator();
 
 					while (it.hasNext()) {
-						HashMap.Entry<String, Object> pair = (HashMap.Entry<String, Object>)it.next();
+						Map.Entry<String, Object> pair = (Map.Entry<String, Object>)it.next();
 						Object child = pair.getValue();
 
 						// If the item is a valid Graph object, add its additional data