title: Empty chain not shown as a choice in selectInput in shiny App
body: <p>I want the user of the App to select the text wrappers for some files they must upload. The options are No Wrapper, Double Quotes and Simple Quotes, so I want the <code>selectInput</code> function to return <code>""</code>, <code>"\""</code> or <code>"'"</code>, but the first choice does not appear. I show a dummy example:</p>
  
  <p>ui.R</p>
  
  <pre><code>shinyUI(fluidPage(
    selectInput("quotes","Select Text wrapper", 
                choices= list("None"="","Double Quotes (\")"='"', "Simple Quotes (\')"="'"), 
                selected="'"),
    verbatimTextOutput("value")
  ))
  </code></pre>
  
  <p>server.R</p>
  
  <pre><code>shinyServer(function(input,output){
    output$value &lt;- renderPrint({ input$quotes })
  })
  </code></pre>
  
  <p>How to make the <code>None</code> choice to appear and to assign an empty character value <code>""</code> to <code>input$quotes</code>? </p>
  
  <p>Note: I know that it works with <code>selectize=FALSE</code>, but I would like to keep the looks of the selectize environment. I also know that you can delete the value and click in the box to get the empty value, but I think this isn't intuitive for the user, I want to have an explicit 'None' choice.</p>
  
  <p>Thanks in advance!</p>