Components: md-icon-button background should be transparent

[savaryt]

Bug, feature request, or proposal:

Inconsistency

What is the expected behavior?

<button md-icon-button>
  <md-icon>some_ligature</md-icon>
</button>

background should be transparent

What is the current behavior?

<button md-icon-button>
  <md-icon>some_ligature</md-icon>
</button>

is not transparent

but

<a md-icon-button>
  <md-icon>some_ligature</md-icon>
</a>

is transparent

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

Inconsistency with the documentation and the behavior of a & button
https://github.com/angular/material2/blob/master/src/lib/button/README.md

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

Alpha9-3

[tinayuangao]

Both button and a have transparent background in current master.

[sothychan]

This appears to longer be true. <button mat-icon-button> is not transparent to me. I have @angular/material-5.2.3 installed.

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