title: Shiny app: downloadHandler does not produce a file
body: <p>I can't figure out what's going on - everything seems to work but my app does not generate a file - although it looks like it does.
  I run it in Windows, on RStudio 0.98.125, and I run it using the line:
  runApp()
  Below is a very simple reproducible example:</p>
  
  <h3>my 'ui.R':</h3>
  
  <pre><code>shinyUI(pageWithSidebar(
  
    headerPanel("My App"),
  
    sidebarPanel(
      numericInput('NumRuns','Number of runs',value=3,min=3,max=10,step=1),
  
      actionButton(inputId="goButton","Run!"),
  
      textInput("downloadData","Save My Data Frame:",value="Data Frame 1"),
      downloadButton('downloadData','Save my file!')
  
    ),
  
    mainPanel(
      tabPanel("Some Text",
               h4(textOutput("caption2")),
               tableOutput("mydf"),
               value=3))
    ))
  </code></pre>
  
  <h3>my 'server.R':</h3>
  
  <pre><code>shinyServer(function(input,output){
  
    # Creating files for download at the end
  
    myout = reactive({
      if(input$goButton==0) return(NULL)
  
        nrruns=input$NumRuns
        mylist=NULL
        for(i in 1:nrruns){
          mylist[[i]]&lt;-data.frame(a=rnorm(10),b=runif(10))
          names(mylist)[i]&lt;-paste("dataframe",i,sep="")
        }
        return(mylist)
    })
  
       output$mydf &lt;- renderTable({
       if(input$goButton==0) return(NULL)
       input$goButton
       isolate(
         myout()$dataframe1
       )
     })
  
    output$downloadData &lt;- downloadHandler(
      filename = function() { paste(input$downloadData, " ",Sys.Date(),".csv",sep="") },
      content = function(file) {
        write.csv(myout()$dataframe1,file,row.names=F)
      }
    )
  
  })
  </code></pre>