Fixing serialization issue when using @PaxWicketBeanFilter

Hi,

It would appear that when using @PaxWicketBeanFilter, the instance of Filter that is created is stored as a field in OSGiServiceRegistryProxyTargetLocator.  As this class is not serializable, it creates a NotSerializableException.

This fix simply keeps the Filter as a constructor argument to ensure that the filter string is legal, but stores the string rather than the Filter instance, avoiding the NotSerializableException.

Cheers,

Ben