[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?

リンクされた StackBlitz の再現から始め、mat-icon と Attach File テキストを含む mat-tab-label テンプレートを調べます。その配置を報告されているボタンの動作と比較し、タブのコンテンツの動作を変更せずに、アイコンとラベルが垂直方向の中央に配置されていることを確認します。

索引モデルが issue の本文から書いたものです。