md-button-toggle-group trigger change event when it should not

[MatthieuBarthel]

Bug, feature request, or proposal:

Bug I think

What is the current behavior?

First of all, I am using a button-toggle-group as a form control, I am not sure that it is officially supported but I guess it is as the doc says it can behave as radio-buttons (and it works).

My issue is that this button-toggle-group triggers a change event when the formControl's value is updated with the setValue method. It is not the case with others material2 form controls (md-input, md-radio-group, md-select, etc.).

What is the expected behavior?

Not triggering the change event.

What are the steps to reproduce?

Here is a Plunker: http://plnkr.co/edit/N1HhjFtJiliGX0z9L2nT?p=preview
Simply click on the button on the top and you will see various material controls being updated, the latest (button-toggle-group) is the only one to trigger a change event.

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

Material beta8 (+ latest snapshot), Angular 4.x, probably any browser.

Is there anything else we should know?

I love material2, thanks for the hard work !

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