Ignore unspecified attributes and changes in length.

Due to the fact that the `attributes` array is live and (in IE8-) contains many unspecified attributes, they are sometimes enumerated twice.  By ignoring unspecified attributes, I think we can work around this issue.

*NB: The `specified` property indicates that the attribute was provided via HTML, though it is not documented in any sane place that I can find.  TMYK.* :stars:

*Edit: Found the [spec](http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-637646024).*