stringio.c: use rb_str_append

* ext/stringio/stringio.c (strio_write): use rb_str_append to
  reuse coderange bits and keep taintedness.
  [ruby-dev:48118] [Bug #9769]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45670 b2dd03c8-39d4-4d8f-98ff-823fe69b080e