Error#to_s should ensure that the value returned is a string.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@30455 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

error.c

@@ -524,8 +524,7 @@ exc_to_s(VALUE exc)
     VALUE mesg = rb_attr_get(exc, rb_intern("mesg"));
 
     if (NIL_P(mesg)) return rb_class_name(CLASS_OF(exc));
-    OBJ_INFECT(mesg, exc);
-    return mesg;
+    return rb_String(mesg);
 }
 
 /*