Update ShowExceptions to suit latest rack master

* Post 893a2c50 in rack/rack, the #pretty method used while generating
  the HTML error markup, implemented in Rack::ShowExceptions, returns a
  String instead of an Array. This change uses Array() to
  convert the exception string in both plain text and HTML modes
  to an array.

* Should fix #906
FYI `pretty()` is not meant to be public so this may bite us again, but I'm ok with merging it for now.