The only remaining use of this function was to get the internal
message object from an exception's hidden `mesg` instance
variable to allow it to be dumped wiithout converting to a string.

As discussed in #103, this exposes internal implementation details
of CRuby, and ultimately does not provide any real utility to the
user since they can't directly inspect this hidden variable. The
test change here is to reflect CRuby behavior that denies equality
if the internal message objects do not match, as is the case after
the exception has been loaded and now has a simple String value.

The impact to users is that exceptions with special hidden message
objects will convert those objects to String during marshaling
through YAML. I believe this only affects NameError and its
descendants, since users can't set this field directly on their
own exception types.

Fixes #103.