config import should use array_diff_key

Although I should be all means refuse to touch config import, it affects my performance work so I need to fix it. Here's array_diff_assoc:

Two values from key => value pairs are considered equal only if (string) $elem1 === (string) $elem2 . In other words a strict check takes place so the string representations must be the same.

This string cast of arrays throws a notice on 5.4 and it's good it does because this is the wrong function to use here. We are not interested in the values at all, we want to get things to delete and things to create, right? Now, that is array_diff_key and not array_diff_assoc. Of course, knowing PHP is less important than lying about what I said.

Here's a patch.