Use jQuery.fn.prop to set a property of an element (instead of jQuery.fn.attr)

[marc-mabe]

It's deprecated and triggers a note since jquery.migration

[DataTables]

Thanks very much for the pull request. The way the length menu is built up in 1.10 is actually slightly different, and I've used val rather than attr or prop. As 1.9.x is no longer being maintained (in the sense that there won't be another 1.9.x release - all changes are going into 1.10), I'm actually not going to pull this request in to encourage uptake of 1.10 when it is released.