#499 - Adding id property to Ext to conform to ALPS spec.

Adapted unit tests to include the additional field.

Original pull request: #500.

src/main/java/org/springframework/hateoas/alps/Ext.java

@@ -29,6 +29,7 @@ import lombok.Value;
 @Builder
 public class Ext {
 
+	private final String id;
 	private final String href;
 	private final String value;
 }