1.9.3 does not have rb_exc_new_str().  use rb_exc_new3() instead of it.
this fixes the test failures introduced by previous commit.


git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/branches/ruby_1_9_3@44755 b2dd03c8-39d4-4d8f-98ff-823fe69b080e