bug report #501832 by andreiashu,AlexisWilke:

In response to post 21, the module now strips collapse
open and close tags that are left over after processing.
This will remove them when they are there in error or if
they are there because of the teaser.
I also added support to let you escape a collapse tag in case
you want it show show up verbatim (e.g. \[collapse...])