Fix: utf8 safe split for Ruby version major than 1.9

This fix spec:

RiCal::Component::Calendar RiCal::Component::Calendar::FoldingStream #utf_safe_split should not split a 2-byte utf character
RiCal::Component::Calendar RiCal::Component::Calendar::FoldingStream #utf_safe_split should not split a 3-byte utf character

With Ruby 2.1.0dev (2013-03-02 trunk 39551) [x86_64-linux]

Thanks