pandoc warning during exercise knit

[jjallaire]

I am seeing this when running exercises:

[WARNING] This document format requires a nonempty <title> element.
  Please specify either 'title' or 'pagetitle' in the metadata.
  Falling back to 'exercise.knit.utf8'

We render exercises using a couple of passes here:

https://github.com/rstudio/learnr/blob/master/R/exercise.R#L199-L244

@yihui Can you recommend the right workaround to suppress this warning? (or do I recall a recent commit to rmarkdown that would get rid of it globally?)

[yihui]

Adding pandoc_args = c("--metadata", "pagetitle=PREVIEW")) to html_fragment() here should fix it (PREVIEW here can be an arbitrary non-empty string):
https://github.com/rstudio/learnr/blob/master/R/exercise.R#L207-L209

[jjallaire]

Okay, @bborgesr can you take a shot at that?

[jcheng5]

@jjallaire She's out this week. I'll submit a PR momentarily.