fix string_spec.rb
fix string_spec.rb

RyanScottLewis/mruby-rubyffi-compat,fundamental/mruby-rubyffi-compat,fundamental/mruby-rubyffi-compat,schmurfy/mruby-rubyffi-compat,schmurfy/mruby-rubyffi-compat,RyanScottLewis/mruby-rubyffi-compat,fundamental/mruby-rubyffi-compat,RyanScottLewis/mruby-rubyffi-compat,schmurfy/mruby-rubyffi-compat