RSM fails to validate certain valid elements

`org.xmpp.resultsetmanagement.ResultSet#isValidRSMRequest` checks if the Element that has been passed as an argument is a valid Result Set Management element, in a request context.

The current implementation returns 'false' if the element does not have a child-element named 'max'. This element is optional in https://xmpp.org/extensions/xep-0059.html

Tinder should not fail on RSM requests that have no 'max' element.

Additionally, a RSM request should be allowed to have both a 'after' and 'before' element.

Combining the 'index' element with 'after' or 'before' seems to mix incompatible paging options. The existing code does not allow for this, which, unless evidence to the contrary is given, seems appropriate.