Add a spec for rb_define_class_under when given a mismatched superclass

Author: ahmadsherif

optional/capi/class_spec.rb

@@ -239,6 +239,10 @@
 
       ClassUnderAutoload.name.should == "ClassUnderAutoload"
     end
+
+    it "raises a TypeError if class is defined and its superclass mismatches the given one" do
+      lambda { @s.rb_define_class_under(CApiClassSpecs, "Sub", nil) }.should raise_error(TypeError)
+    end
   end
 
   describe "rb_define_class_variable" do