bug(md-select): options not sized correctly if no placeholder set

[macjohnny]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When using a md-select without the placeholder attribute, the options shown should be the same width as the md-select element.

What is the current behavior?

The options are at a smaller width than the md-select attribute, since the _triggerWidth property is only set when the placeholder attribute is set, which is done in https://github.com/angular/material2/blob/master/src/lib/select/select.ts#L217

What are the steps to reproduce?

http://embed.plnkr.co/U9jHgw/

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

@angular/material: 2.0.0-beta.2
Google Chrome Version 56.0.2924.87 (64-bit)

[macjohnny]

a workaround is setting the placeholder attribute to a space, i.e. placeholder=" "

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