[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?

Commencez par la reproduction StackBlitz liée et inspectez le modèle mat-tab-label contenant mat-icon et le texte Attach File. Comparez son alignement avec le comportement signalé du bouton, puis vérifiez que l’icône et le libellé sont centrés verticalement sans modifier le comportement du contenu de l’onglet.

Rédigé par le modèle d'indexation à partir du texte de l'issue.