Issue #2365469: Add default values.
Issue #2365469: Ensure max-age / downstream-ttl is converted to array.
Issue #2365469: Remove empty properties to avoid confusing drupal_render().
Issue #2365469: Ignore code coverage for PHPUnit.
Issue #2365469: Improve test coverage for stack.
Issue #2365469: Use a better way for non-supports dynamic assets case.
Issue #2365469: Add remaining test coverage.