Get array of all POST data with set_value()

This allows the user to pass a NULL value to `set_value()` in order to receive an array of all the POST data that was validated. 

Use it like this: 

`print_r(set_value(NULL));`
Gives you 
    Array
    (
        [name] => David Marshall Lemcoe Jr.
        [data] => More data
    )

This mirrors the functionality of `$this->input->post(NULL);`