title: special symbols in value field of textInput
body: <p>I would like to use things like greek letters and subscripts in the value field of a textInput widget. I know how to do this for the label but the same thing doesn't work in the field. Here is  toy example</p>
  
  <pre><code>    runApp(list(
    ui = fluidPage(
      uiOutput("txt")
    ),
    server = function(input, output) {
      output$txt &lt;- renderUI({
         textInput("a", 
              label = HTML(" &amp;pi;&lt;sub&gt;0&lt;/sub&gt; works here"), 
              value = HTML("but not &amp;pi;&lt;sub&gt;0&lt;/sub&gt; here"))
  
      })    
    }  
  ))
  </code></pre>
  
  <p>This displays the the greek letter and the subscript properly in the label but not in the text field</p>