[Tabs] Header with icon+text not vertically aligned

#### Bug, feature request, or proposal:
Tab with Icon and Text label not aligned vertically center, currently aligned bottom

#### What is the expected behavior?
Expect to align text and icon in middle. (as In Button with Icon and Label)
Tab Label and Icon are aligned in

#### What is the current behavior?
Tab with Icon and Text label not aligned vertically center, currently aligned bottom

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

https://stackblitz.com/angular/rlqokrmnevel?file=app%2Ftabs-template-label-example.html

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attach_file Attach File

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#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@5.2.2

#### Is there anything else we should know?

Start with the linked StackBlitz reproduction and inspect the mat-tab-label template containing mat-icon and the Attach File text. Compare its alignment with the reported button behavior, then verify that the icon and label are vertically centered without changing tab content behavior.

Written by the indexing model from the issue text.