Module#append_features should raise an exception when other is a frozen module

### Example of MRI behavior

MRI(ruby 1.8.7 (2012-10-12 patchlevel 371))

```ruby
Module.new.freeze.send :include, Enumerable
  #=> :in `append_features': can't modify frozen module (TypeError)
```

MRI(ruby 1.9.3p429 (2013-05-15 revision 40747)

```ruby
Module.new.freeze.send :include, Enumerable
  #=> in `append_features': can't modify frozen module (RuntimeError)
```