AjaxRequestAttributes extra parameters aren't properly handled in getCallbackFunction()

extra parameters of an Ajax behaviour can be accessed by getRequest().getRequestParameters().getParameterValue(key)

but if one uses getCallbackFunction() of an AbstractDefaultAjaxBehavior, these parameters get screwed and can no longer be accessed in the same manner.

the problem seems to be the merge in attrs.ep = Wicket.merge(attrs.ep, params);