Fix for setting NULL

In different cases if you set NULL to something, then it is checked
with `isset()` in changed, content and other arrays when read.

But in PHP

``` php

$arr = array('key' => NULL);
isset($arr['key']); // FALSE, becase
isset(NULL); // FALSE
```

So if you set NULL to association, then it does not count it as changed.

Changed things:

- `_content` in `Jam_Array` is checked with `array_key_exists`
- `Jam_Model::get()` now uses `array_key_exists` to check for changes in associations similar to `Jam_Validated::get()` for fields.
- `Jam_Validated::unmapped()` now uses `array_key_exists` to check for unmapped data.