force UTF-8 if it's ASCII-8BIT (#180)
force UTF-8 if it's ASCII-8BIT (#180)

ASCII-8BIT implies that it’s been read from the file system; in that
case, let’s assume UTF-8. I think this should really be picking up the
encoding from the dialect but I’m wary about fiddling around too much
strategicdata/csvlint.rb,ResultadosDigitais/csvlint.rb,nickzoic/csvlint.rb,strategicdata/csvlint.rb,theodi/csvlint.rb