Splat operator to frozen Array raises FrozenError

I tried to use splat operator when appends value to the frozen array, but it raises FrozenError in mruby 2.1.2.  
I'll expect to get similar results to CRuby.

```ruby
x = [1, 2].freeze
y = [*x, 3]
p y
```

```
% ruby a.rb 
[1, 2, 3]
% mruby a.rb
can't modify frozen Array (FrozenError)
```