Removes #to_str implementation from test_double

Having `test_double` implement `to_str` makes the Ruby runtime coerce it to a string whenever needed and this isn't the intended behaviour for this object. This behaviour led to issue #567 as Ruby will try to coerce the exception object into string when raising ( [here](https://github.com/ruby/ruby/blob/191b373d26a48058bcc8955bf3afe426f60eaaea/eval.c#L678) ) an exception and since `test_double` does implement `to_str` it is coerced to string and raised instead of failing with the correct 'this is not an exception object' error.

This fixes #567 by providing a better error message in it's case.