panelset incorrectly applies panel classes

[gadenbuie]

::: panelset
::: {.panel name="Install" .border .border-1}
Install xaringanExtra from [CRAN][cr] in the usual way.

```{.r}
install.packages("xaringanExtra")

Or install the development version from [GitHub][gh] or [r-universe][ru].

pak::pak("gadenbuie/xaringanExtra")

install.packages("xaringanExtra", repos = c("https://gadenbuie.r-universe.dev", "https://cran.r-project.org"))

::: :::

currently results in

```html
<section class="panel panel,border,border-1 panel-active" id="install" role="tabpanel" aria-labelledby="install" tabindex="0" style="">
  <!-- panel stuff -->
</section>

but should be

<section class="panel border border-1 panel-active" id="install" role="tabpanel" aria-labelledby="install" tabindex="0" style="">
  <!-- panel stuff -->
</section>