Java (Eclipse) compatibility of extensions, see #3469

* Eclipse JDT showed compilation error
  "The method get(ActorSystem) is ambiguous for the type..."
  for extensions defined with ExtensionKey

akka-docs/rst/java/code/docs/extension/ExtensionDocTest.java

@@ -35,6 +35,8 @@ public class ExtensionDocTest {
     //This will be the identifier of our CountExtension
     public final static CountExtension CountExtensionProvider = new CountExtension();
 
+    private CountExtension() {}
+
     //The lookup method is required by ExtensionIdProvider,
     // so we return ourselves here, this allows us
     // to configure our extension to be loaded when