Add testing functionality [mod_download_figure]

[espors]

What

Add testing functionality

• test that figure() is an object
• provide notes if the correct information is not entered
• add other information??

[bderenge]

In addition to testing if figure is an object, we could take steps to ensuring the object has the correct state, and detecting any dependencies it before the image is downloaded . For example, I found a bug where a ggplot object call p is not reactive, but if I pass p into the module with parentheses, like this: p <- ggplot(...) + geom_histogram()

  dl_hist_r <- mod_download_figure_server(
    "dl_hist_r", 
    filename = "histogram_r", 
    figure = reactive({ p() })
  )
  the module appears to work perfectly downloading a pdf with the correct name and extension. But the pdf is actually corrupted and will not open. This was resolved by removing the parentheses by p.
  dl_hist_r <- mod_download_figure_server(
    "dl_hist_r", 
    filename = "histogram_r", 
    figure = reactive({ p })
  )
  It was my mistake using p() instead of p, but looking at the code for the demo example, _figure_ appears to be a reactive object inside another reactive call, even though the package also works by using a non-reactive object in the reactive call.  The error comes from treating a non-reactive as something reactive, p() instead of p, which is something we could test for.