Make default reactive domain available in request handlers. Fixes #669

Progress bar now works properly with an app like this:

```R
shinyApp(
  ui = bootstrapPage(
    downloadLink('download')
    # textOutput('txt')
  ),
  server = function(input, output, session) {
    output$download <- downloadHandler(
      'foo.txt',
      function(file) {
        withProgress({

          for (i in 1:5) {
            incProgress()
            Sys.sleep(0.2)
          }

          writeLines("test file", con = file)
        })
      }
    )
  }
)
```