Stepper icon override not aligned correctly

[kgajera]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Centered alignment when overriding icon like this (as shown in the current docs):
<ng-template matStepperIcon="number" let-index="index"> {{index + 10}} </ng-template>

What is the current behavior?

Aligned to the top left

What are the steps to reproduce?

https://stackblitz.com/edit/angular-xeo5ty

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

@angular/material@6.4.7

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