stringio.c: use rb_str_append other than ASCII-8BIT
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45677 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
stringio.c: use rb_str_append other than ASCII-8BIT
reuse coderange bits other than ASCII-8BIT, and keep
taintedness. [ruby-dev:48118] [Bug #9769]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45677 b2dd03c8-39d4-4d8f-98ff-823fe69b080e