Force raising TypeError on Ruby 1.9.2 when raising invalid object

This forces the same behavior of other rubies in 1.9.2 when
trying to raise an object that is not an exception and can not
be coerced to string. Since 1.9.2 will not call respond_to?(:to_str)
the double never had a chance to say it did not implement that
and method_missing was called right away generating an unexpected
message error.