mat-select should enable specifying aria-label without specifying placeholder

[mertdeg2]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

User can specify [aria-label] attribute without specifying [placeholder]

What is the current behavior?

[placeholder] overrides the aria-label attribute

What are the steps to reproduce?

http://plnkr.co/edit/yPrSiGr1mx4c1vW4sKsO?p=preview

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