Fix header not disappearing after table out of view

When using with useAbsolutePositioning: false and given scrollContainer like this:

    $('.floathead]').floatThead(
      scrollContainer: ($tbl) -> $tbl.closest($tbl.data('scroll'))
      useAbsolutePositioning: false
    )
Merge pull request #82 from clyfe/master

Fix header not disappearing after table out of view