BUGFIX: Return a RootForm for AssetAdmin so Form get the correct Fields on POST
BUGFIX: Return a RootForm for AssetAdmin so Form get the correct Fields on POST

Solves issues with GridField and uploading on the root folder (/assets/) 
dhensby/silverstripe-cms,dhensby/silverstripe-cms,dhensby/silverstripe-reports,silverstripe-labs/silverstripe-reports,jonom/silverstripe-cms,dhensby/silverstripe-reports,dhensby/silverstripe-cms,silverstripe/silverstripe-cms,silverstripe/silverstripe-cms,silverstripe-labs/silverstripe-reports,silverstripe/silverstripe-siteconfig,helpfulrobot/i-lateral-silverstripe-siteconfig,jonom/silverstripe-cms