Removes #to_str implememtation 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 behavior for this object. This behavior 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 succeded and raises that 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.