string.c: don't clear coderange at expanding

* string.c (rb_str_modify_expand): like as str_modify_keep_cr,
  don't clear coderange unless broken.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45622 b2dd03c8-39d4-4d8f-98ff-823fe69b080e