Implement Array#concat per RubySpec

```
topaz (ruby-1.9.3p125) [x86_64-darwin]

Array#concat
- returns the array itself
- appends the elements in the other array
- does not loop endlessly when argument is self
- tries to convert the passed argument to an Array using #to_ary
- does not call #to_ary on Array subclasses
- raises a RuntimeError when Array is frozen and modification occurs
- raises a RuntimeError when Array is frozen and no modification occurs
- keeps tainted status
- is not infected by the other
- keeps the tainted status of elements
- keeps untrusted status
- is not infected untrustedness by the other
- keeps the untrusted status of elements


Finished in 1.655285 seconds

1 file, 13 examples, 27 expectations, 0 failures, 0 errors
```