title: Export function does not support .svg downloading
body: <p>I have the shiny app below which displays a <code>plotly()</code> oblect and a download button to download it. While png and jpeg are downloaded normally there seems to be an issue when I try to download the svg file. </p>
  
  <pre><code>library(shiny)
  library(plotly)
  library(webshot)
  ui &lt;- fluidPage(
    plotlyOutput("plot"),
    uiOutput("down")
  )
  
  server &lt;- function(input, output) {
  
    # renderPlotly() also understands ggplot2 objects!
    save&lt;-reactive({
      plot_ly(mtcars, x = ~mpg, y = ~wt)
  
    })
    output$plot &lt;- renderPlotly({
      save()
    })
  
    output$down&lt;-renderUI({
  
      #Download files with quotes or not depending on the quote=input$quotes which has value TRUE or FALSE.
      output$downloadData &lt;- downloadHandler(
        filename = function() {
          paste("test", "svg", sep = ".")
        },
  
        # This function should write data to a file given to it by
        # the argument 'file'.
        content = function(file) {
          # Write to a file specified by the 'file' argument
          export(save(), file=file)
  
        }
  
      )
      downloadButton("downloadData", "Download",class = "butt1")
    })
  }
  
  shinyApp(ui, server)    
  </code></pre>