Improve canonicalizeHtml to ignore internal Blaze attributes

On IE8, attributes are serialized into innerHTML. Ignore
the $meteor_ui_removal_callbacks attribute that's used
to keep track of callbacks to fire when an element is removed.
Update DomBackend tests for IE8 removal behavior

In IE8, removing an element from the DOM makes it
have a parentNode which is a document fragment
that itself has no parentNode.

This is different than on other browsers that leave
the element with no parentNode.