[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

```

attach_file Attach File

```

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

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

从链接的 StackBlitz 复现开始，检查包含 mat-icon 和 Attach File 文本的 mat-tab-label 模板。将其对齐方式与报告中的按钮行为进行比较，然后验证图标和标签是否垂直居中，同时不改变选项卡内容的行为。

由索引模型根据 Issue 内容生成。