fix RemoteFileBuffer.truncate() when size is None
fix RemoteFileBuffer.truncate() when size is None

pmitros/pyfs,Konubinix/pyfilesystem,PyFilesystem/pyfilesystem,pombreda/pyfilesystem,flying-circus/pyfilesystem,rivenz/pyfilesystem,pmitros/pyfs,piranna/pyfilesystem,zopyx/pyfilesystem