fix warning when uploading file using webdav

When uploading a file using davfs php warned about a missing argument to
OC_FileProxy_Quota::preFile_put_contents(). Since we get a resource from
OC_Connector_Sabre_File->put(), we have to convert that before running
strlen because it expects a string.

Signed-off-by: Florian Pritz <bluewind@xinu.at>