Nested expansion panel have irrelevant margin.

[collonian]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Nested expansion panel should have margin.

What is the current behavior?

Expansion panel doesn't have margin top or bottom when it is nested in the first or last expansion panel.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-ljxt4s

When I Open '01' or '21' panel, it doesn't have margin-top or margin-bottom.
'01' or '21' panel should looks like '11' panel.

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

It should be displayed consistent.

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

Angular: 6.0
Material: 6.0
OS: Windows 10
TypeScript: 2.7.2
Browser: Chrome

Is there anything else we should know?

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