fix(dialog): RTL does not have 8dp padding between side-by-side buttons

[robzenn92]

This issue is related to #661 which introduces 8dp padding between side-by-side buttons but does not take into account the RTL context.

What MDC-Web Version are you using?

0.12.0

What browser(s) is this bug affecting?

All of them

What OS are you using?

macOS 10.12.4

What are the steps to reproduce the bug?

1. Open http://localhost:8080/dialog.html or visit the demo page
2. Check on Toggle RTL

What is the expected behavior?

There should be 8dp padding between side-by-side buttons

What is the actual behavior?

There is no 8dp padding between side-by-side buttons.

[robzenn92]

This should be solved using mdc-rtl-reflexive-box within line L137 as follows:

&__button {
  @include mdc-rtl-reflexive-box(margin, right, 8px);

  &:last-child {
    @include mdc-rtl-reflexive-box(margin, right, 0);
  }
}