MdIcon: SVG icon is not removed when svgIcon input is set to undefined

[benelliott]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When you render an SVG icon using MdIcon's svgIcon input then clear the input, the SVG element should be removed from the DOM.

What is the current behavior?

The SVG element stays in the DOM.

What are the steps to reproduce?

1. Create an md-icon with svgIcon
2. Set svgIcon to undefined
3. See that the icon is not cleared

Providing a Plunker (or similar) is the best way to get the team to see your issue.
See: http://plnkr.co/edit/Rd7LEU0nDBYoogHyfZQV?p=preview

What is the use-case or motivation for changing an existing behavior?

Cannot reuse an MdIcon instance for configurations that do or do not use svgIcon.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material 2.0.0-beta.8, Angular 4.3.3, TypeScript 2.4.2

Is there anything else we should know?

As a temporary workaround you can use two MdIcon instances, but predicate them on whether the icon source is an SVG and swap them out when necessary.

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