[mat-card mat-divider] [bug] make mat-divider css more specific or isolate

[wilker7ribeiro]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

mat-divider CSS from mat-card should not have influence in mat-dividers inside mat-card-content

What is the current behavior?

All mat-dividers inherit's this CSS above, breaking their behavior (0px height)

.mat-card .mat-divider {
    position: absolute;
    left: 0;
    width: 100%;
}

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-tml6fe

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

@angular/core@6.0.6
@angular/materia@6.3.0

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