Race condition where clientData$singletons can be NULL

Reproduced in Shiny Server load testing.

Code reviewed by @trestletech
add hr tag to exported html builder tags
update NEWS
Merge pull request #324 from rstudio/feature/hr-tag

add hr tag to exported html builder tags
merge from master

Merge remote-tracking branch 'origin/master' into feature/bootstrap-layout

Conflicts:
	NAMESPACE
	NEWS