[md-slide-toggle] setting default value to true brings pristine: false

[e-cloud]

Bug, feature request, or proposal:

BUG

What is the expected behavior?

default value with true should not affect the pristine state

What is the current behavior?

pristine state is changed

What are the steps to reproduce?

http://plnkr.co/edit/cCqjrY62mF1WKBSFPWeH?p=preview

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

Current behavior is somehow disturbing form validation

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

Angular-4.1.0, Material-2.0.0-beta-6, others are all

[e-cloud]

see #4220

[csvn]

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

New releases of material broke plunker. This one shows the issue.

[devversion]

This should be fixed with dfe10c3

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