Fix post_select multi select with AJAX

Damn theres so many possible combinations of stuff to test. I don't think this was ever tested before! It simply can never have worked.

* No need to append [] to the name when using standard input. In fact this causes an error on save trying to explode an array.
* Thats not how you use push in JS!
* When outputting data in initSelection - Completely wrong var used in foreach for getting the post id!
* Exploding an empty string will save an empty array - when we want to delete the meta instead.

Easy #review #merge this one