readline.c: use rb_setup_fake_str

* ext/readline/readline.c (readline_s_delete_text): initialize a
  fake string by rb_setup_fake_str().

* internal.h (rb_setup_fake_str): allow extensions to call.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47555 b2dd03c8-39d4-4d8f-98ff-823fe69b080e