touch device (e.g. iOS) dropdown backdrop is removed incorrectly

Created by: gwynjudd

This issue can be seen in this fiddle (http://jsfiddle.net/rfyt2fr3/1/) on an iOS touch device (probably other touch devices as well):

1. click on the dropdown to show it
2. verify using the web inspector that the dropdown backdrop has been added
3. click anywhere in the page
4. note that the dropdown backdrop is removed, but the dropdown didn't close due to the js event handler

On touch devices, the bootstrap plugin adds a dropdown:

https://github.com/twbs/bootstrap/blob/master/js/dropdown.js#L69

      if ('ontouchstart' in document.documentElement && !$parent.closest('.navbar-nav').length) {
        // if mobile we use a backdrop because click events don't delegate
        $(document.createElement('div'))
          .addClass('dropdown-backdrop')
          .insertAfter($(this))
          .on('click', clearMenus)
      }

It's evident that this backdrop is meant to compensate for the known issue of click events not propagatingdelegating, otherwise the clearMenus doesn't run when you click anywhere:

https://github.com/twbs/bootstrap/blob/master/js/dropdown.js#L158

  $(document)
    .on('click.bs.dropdown.data-api', clearMenus)

The code that removes the backdrop does so, even if the hide.bs.dropdown event handler prevented the close.

https://github.com/twbs/bootstrap/blob/master/js/dropdown.js#L37

  function clearMenus(e) {
    if (e && e.which === 3) return
    $(backdrop).remove()