title: renderDataTable in Shiny not rendering output correctly
body: <p>I am executing a very minimal <code>renderDataTable</code> example posted on the DT github page below
  <a href="http://rstudio.github.io/DT/shiny.html" rel="nofollow noreferrer">http://rstudio.github.io/DT/shiny.html</a></p>
  
  <pre><code>library(shiny)
  shinyApp(
    ui = fluidPage(DT::dataTableOutput('tbl')),
    server = function(input, output) {
      output$tbl = DT::renderDataTable(
        iris, options = list(lengthChange = FALSE)
      )
    }
  )
  </code></pre>
  
  <p>However, the output I am seeing after executing this code is garbage, column names listed in a single line without space.</p>
  
  <p><a href="https://i.stack.imgur.com/Xs1nk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Xs1nk.png" alt="enter image description here"></a></p>
  
  <p>I tried this after uninstalling my DT packages and reinstalling again </p>
  
  <pre><code>devtools::install_github('rstudio/DT')
  </code></pre>
  
  <p>Nothing changed, still the same results. I don't understand why  DT::renderDataTable() is not working. Any suggestions is greatly appreciated.?</p>
  
  <p>-------------Update----------------</p>
  
  <p>I started noticing this issue after I started building some shinyapps using Flexdashboard. Before installing Flexdashboard package everything was working as usual and there was no problem, after installing Flexdashboard i noticed this issue with datatables while using <code>renderDataTable</code> function</p>