fix(a): don't preventDefault on click when SVGAElement has an xlink:href attribute

Before this change, an SVGAElement with an xlink:href attribute and no href or name attribute which was compiled by the angular HTML compiler would never be clickable, due to the htmlAnchorDirective calling event.preventDefault() due to the missing href attribute.

This change corrects this behaviour by also testing the xlink:href attribute if the element in question is determined to be an SVG anchor tag (with the href property having type SVGAnimatedString)

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Some proper E2E testing would test this fix much more effectively.

This is demonstrated to be working [here](http://run.plnkr.co/iGaJgCfhYp1IRa3F/) ([edit here](http://plnkr.co/edit/Ypu8jihgWtE8rbYO64Tg?p=info), but the link won't work within the iframe, only outside it)
Closes #5896