in request.rb (line 503) you'll find
quoted_boundary = Regexp.quote(boundary, "n")
using the super secret never documented second argument to Regexp.quote. Ruby 1.9 has done away with that second argument. I couldn't find much about what this argument does (my best guess I could find was that it sets what multibyte handling to use, with n being none).
Does this argument do something useful here (i.e. is a problem that it's gone in 1.9? no tests failed when I removed it) ?