[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?

Inizia con la riproduzione StackBlitz collegata e ispeziona il template mat-tab-label contenente mat-icon e il testo Attach File. Confronta il suo allineamento con il comportamento segnalato del pulsante, quindi verifica che l'icona e l'etichetta siano centrate verticalmente senza modificare il comportamento del contenuto della scheda.

Scritto dal modello di indicizzazione a partire dal testo della issue.