string.c: pool only bare strings in fstring

* 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

ext/-test-/string/fstring.c

@@ -0,0 +1,15 @@
+#include "ruby.h"
+
+VALUE rb_fstring(VALUE str);
+
+VALUE
+bug_s_fstring(VALUE self, VALUE str)
+{
+    return rb_fstring(str);
+}
+
+void
+Init_fstring(VALUE klass)
+{
+    rb_define_singleton_method(klass, "fstring", bug_s_fstring, 1);
+}