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/SettingsExtensionDocTest.java

@@ -44,6 +44,8 @@ public class SettingsExtensionDocTest {
     implements ExtensionIdProvider {
     public final static Settings SettingsProvider = new Settings();
 
+    private Settings() {}
+
     public Settings lookup() {
       return Settings.SettingsProvider;
     }