Prevent duplicate files as arrays due to race conditions

If one request successfully writes out the agrcache variable while another request is trying to set the same variable with the same file in it, the value of the file gets turned into an array by array_merge_recursive(). Take the example here:

$a = array(
  'dcbf43e6462bd3457745969ce70a77aed363228644780f0457f992212126f91b' => 'public://js/js_xcPFtQGZ355MfHOMBpub2dvRzpcGLjku_Cr0w3wLtwU.js',
  '1e77d096f760f8e00283f36bdce62d2c6210a872b0fc03505396c33d0fe4c04c' => 'public://js/js_x6kTdqbY_yj4pGofJJKAIw1d6so4lTKH5eEZfib4Jec.js'
);


$b = array(
  '35c289ea3bff928ee87ad0ff6f798e68a77cdb67a07d8fbd3cdfded2cde0fc96' => 'public://js/js_v4Kx_zj247divDgyI-9uO91P4Roag8yzWe-1_8c6LWU.js',
  '1e77d096f760f8e00283f36bdce62d2c6210a872b0fc03505396c33d0fe4c04c' => 'public://js/js_x6kTdqbY_yj4pGofJJKAIw1d6so4lTKH5eEZfib4Jec.js'
);

$c = array_merge_recursive($a, $b);
print_r($c);

This will output:

Array
(
    [dcbf43e6462bd3457745969ce70a77aed363228644780f0457f992212126f91b] => public://js/js_xcPFtQGZ355MfHOMBpub2dvRzpcGLjku_Cr0w3wLtwU.js
    [1e77d096f760f8e00283f36bdce62d2c6210a872b0fc03505396c33d0fe4c04c] => Array
        (
            [0] => public://js/js_x6kTdqbY_yj4pGofJJKAIw1d6so4lTKH5eEZfib4Jec.js
            [1] => public://js/js_x6kTdqbY_yj4pGofJJKAIw1d6so4lTKH5eEZfib4Jec.js
        )

    [35c289ea3bff928ee87ad0ff6f798e68a77cdb67a07d8fbd3cdfded2cde0fc96] => public://js/js_v4Kx_zj247divDgyI-9uO91P4Roag8yzWe-1_8c6LWU.js
)

The file gets turned into an array, which fails to be served. I found this bug during a load test and I was only able to recreate it with a high level of concurrency after clearing the variables out.

Attached patch fixes this by just using array_merge on each element which should produce the desired result.