Bug mat-button-toggle: button inherit margin from class in mat-button-toggle

[LoganDupont]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The margin should only be set on the mat-button-toggle

What is the current behavior?

The margin is set on the mat-button-toggle and child button element.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-csuqbl

What is the use-case or motivation for changing an existing behavior?

If a user sets for example a margin-right of 15px. He will get actually 15px on the right on the mat-button-toggle and also 15px on the button element itself.

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

All browsers
Angular 6.0.6
Material 6.3.1

Is there anything else we should know?

Still working on Material 6.3.0

[LoganDupont]

@crisbeto Do I need to create a new issue as the same is happening for padding? I also found the commit with the changes b959e18

[crisbeto]

It's not necessary @LoganDupont, I'll include it in my current PR.

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