Update elFinderVolumeDriver.class.php

if !function_exists('proc_open'):
Notice undefined index 'create' elFinderVolumeDriver.class.php @ line 898
[core:ui:upload] correction of discover URL on URL upload
[core:backend] use ContentType 'application/force-download' on download

changed from 'application/octet-stream' to 'application/force-download'
Merge pull request #745 from Illirgway/patch-2

Update elFinderVolumeDriver.class.php
Merge branch '2.x' into 2.1
Merge branch '2.1' into 2.1_n