md-select change event fires twice on keyboard navigation

[atinybeardedman]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

change event should fire only once per selection

What is the current behavior?

the change event as well as the ngModelChange event fires twice when using the up and down arrows on the keyboard to select a value.

What are the steps to reproduce?

This occurs only when the item is in focus and the keyboard is used (no panel is shown).

Providing a Plunker (or similar) is the best way to get the team to see your issue.

https://plnkr.co/edit/0tnDAA?p=preview

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

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

Angular 4.3.5
Material beta.10
Tested in chrome

Is there anything else we should know?

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