JavascriptReferenceType: Implement Serializable (#664)

* JavascriptReferenceType: Implement Serializable

Implement classes newly added as Serializable to be able to store in cache or to be serialized.

Fix https://issues.apache.org/jira/projects/WICKET/issues/WICKET-7076?filter=allissues

* JavaScriptReferenceType.java: Change from Serializable to Iclusterable

Following discussion, it is better to use IClusterable as it is used in other classes like this.

Fix https://issues.apache.org/jira/projects/WICKET/issues/WICKET-7076?filter=allissues

wicket-core/src/main/java/org/apache/wicket/markup/head/JavaScriptReferenceType.java

@@ -16,13 +16,14 @@
  */
 package org.apache.wicket.markup.head;
 
+import org.apache.wicket.util.io.IClusterable;
 import org.apache.wicket.util.lang.Args;
 
 /**
  * To be used to define the "type" attribute of the script tag written
  * by a {@link AbstractJavaScriptReferenceHeaderItem}.
  */
-public class JavaScriptReferenceType {
+public class JavaScriptReferenceType implements IClusterable {
 
 	public static final JavaScriptReferenceType TEXT_JAVASCRIPT = new JavaScriptReferenceType("text/javascript");
 	public static final JavaScriptReferenceType MODULE = new JavaScriptReferenceType("module");