offset-top=0 and offset-bottom=0 don't work on affix plugin via data attributes

Created by: tarot

Via JavaScript, affix plugin can accept {offset:{top:0, bottom:0}} options.

But via data attributes,

<nav data-spy="affix" data-offset-top="0" data-offset-bottom="0">...</nav>

means

<nav data-spy="affix">...</nav>

Probrem in: https://github.com/twbs/bootstrap/blob/master/js/affix.js#L130-L131

Expected:

      if (data.offsetBottom != null) data.offset.bottom = data.offsetBottom
      if (data.offsetTop != null)    data.offset.top    = data.offsetTop