Improved node-view's _isHidden
See PR 895
`node-view` `_hide` sets `hidden` attribute to empty string
That's what the spec says! See
* http://dev.w3.org/html5/spec-preview/common-microsyntaxes.html#boolean-attributes
* http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-F68F082