* string.c (fstr_update_callback): pool bare strings only.
* string.c (rb_fstring): return the original string with sharing a
  fstring if it has extra attributes, not the fstring itself.
  [ruby-dev:49188] [Bug #11386]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51360 b2dd03c8-39d4-4d8f-98ff-823fe69b080e