title: Add download buttons in DT::renderDataTable
body: <p>I am trying to add download buttons ('copy', 'csv', 'excel', 'pdf') above the table in my R Shiny app, but the renderDataTable seems doesn't work when using a datatable inside.</p>
  
  <pre><code>output$mytable1  &lt;- DT::renderDataTable(
          datatable(
              { plots.dfs()[[1]] },
          rownames = TRUE,
          options = list(
              fixedColumns = TRUE,
              autoWidth = TRUE,
              ordering = FALSE,
              dom = 'tB',
              buttons = c('copy', 'csv', 'excel', 'pdf')
          ),
          class = "display"
      ))
  </code></pre>
  
  <p>When I use DT::renderDataTable without DT::datatable inside, renderDataTable works well and I have all features (filters, search field, etc), except download buttons (what I am trying to add)</p>
  
  <pre><code>output$mytable1 = DT::renderDataTable({ plots.dfs()[[1]] })
  </code></pre>
  
  <p>Do you have any idea of what I am doing wrong? Thanks for your help</p>