Remove private_iv_get

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.

ext/psych/lib/psych/visitors/yaml_tree.rb

@@ -181,7 +181,7 @@ module Psych
       end
 
       def visit_Exception o
-        dump_exception o, private_iv_get(o, 'mesg')
+        dump_exception o, o.message.to_s
       end
 
       def visit_NameError o

ext/psych/psych_yaml_tree.c

@@ -2,23 +2,11 @@
 
 VALUE cPsychVisitorsYamlTree;
 
-/*
- * call-seq: private_iv_get(target, prop)
- *
- * Get the private instance variable +prop+ from +target+
- */
-static VALUE private_iv_get(VALUE self, VALUE target, VALUE prop)
-{
-    return rb_attr_get(target, rb_intern(StringValueCStr(prop)));
-}
-
 void Init_psych_yaml_tree(void)
 {
     VALUE psych     = rb_define_module("Psych");
     VALUE visitors  = rb_define_module_under(psych, "Visitors");
     VALUE visitor   = rb_define_class_under(visitors, "Visitor", rb_cObject);
     cPsychVisitorsYamlTree = rb_define_class_under(visitors, "YAMLTree", visitor);
-
-    rb_define_private_method(cPsychVisitorsYamlTree, "private_iv_get", private_iv_get, 2);
 }
 /* vim: set noet sws=4 sw=4: */

test/psych/test_exception.rb

@@ -154,7 +154,8 @@ module Psych
 
     def test_convert
       w = Psych.load(Psych.dump(@wups))
-      assert_equal @wups, w
+      assert_equal @wups.message, w.message
+      assert_equal @wups.backtrace, w.backtrace
       assert_equal 1, w.foo
       assert_equal 2, w.bar
     end