Solved json obj to array issue

ticket: http://open.silverstripe.org/ticket/8330

What came from cache:
```
{"silverstripe\/cms":"dev-master","silverstripe\/framework":"dev-master"}
```

Json encoded:
```
stdClass Object
(
    [silverstripe/cms] => dev-master
    [silverstripe/framework] => dev-master
)
```

What we expect:
```
Array
(
    [silverstripe/cms] => dev-master
    [silverstripe/framework] => dev-master
)
```