Fix Matrix.rows copy bug

This fixes an issue where the `copy` argument was not properly being passed into `#convert_to_array`.
Thus making it possible for the internal structure of matrix to be mutated from outside the instance.

Thank you @shyouhei for the [git contributing guide](https://github.com/shyouhei/ruby/wiki/noncommitterhowto)

/cc @shyouhei @tenderlove