MD-Checkbox required-Attribute not passed to the underlying element

[Sabartius]

Bug, feature request, or proposal:

md-Checkbox should pass "required" to the inner checkbox

What is the expected behavior?

When marking a md-checkbox with required, the underlying input-element should have a "required" attribute.

What is the current behavior?

the required attribute is not passed to the underlying input-element when using md-checkbox

but when using md-input it works

What are the steps to reproduce?

http://plnkr.co/edit/DbPdlyZrfgv3iZsQUNS4?p=preview

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

The HTML5-Validation of required checkboxes should work, this is especially important when working with indeterminate checkboxes

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

looks like everyone

Is there anything else we should know?

[angular-automatic-lock-bot]

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