json_decode fix

json_decode on line 28 is currently returning a stdClass rather than an array, setting the second argument to true fixes this and allows the array_merge on line 33 to work the second time around once assets:generate has been used.

Also, please see the [Pull request submitted by SerafimArts](https://github.com/CodeSleeve/asset-pipeline/pull/218) as this fixes assets:generate along with this pull request.

Thanks :)