handle multiple newlins/carriage-returns

[kellyredding]

Right now we do:

.gsub(/(\r|\n)+/, Xmlss::Writer::LB)

This unintentionally prevents you from doing multiple newlines back-to-back (the +) squashes them. The goal was to not do multiple newlines for "\r\n".

We need a better regex: something that says if given either a single carriage-return/newline OR the combo.