Resources - Make returned content Collection immutable.

src/main/java/org/springframework/hateoas/Resources.java

@@ -18,6 +18,7 @@ package org.springframework.hateoas;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Iterator;
 
 import javax.xml.bind.annotation.XmlAnyElement;
@@ -100,7 +101,7 @@ public class Resources<T> extends ResourceSupport implements Iterable<T> {
 	 * @return the content will never be {@literal null}.
 	 */
 	public Collection<T> getContent() {
-		return content;
+		return Collections.unmodifiableCollection(content);
 	}
 
 	/*