I've done the following changes:

1. create insert_header(): Which places header at the beginning.
For that I created set() method which actually takes an argument `pos` default to 0.

2. __setitem__() uses set() (new method) method while fixing `pos` to end.
3. _parse_values(), new method created to not duplicate the code in insert_header() and add_header() since both of them merely do same job.

Please let me know your opinions on it.