if output file is -, winpmem is not raw output

[dijiang]

cmd like this "winpmem_v3.3.rc3.exe -dd -o test.raw --format raw --volume_format raw" , it's raw output. but if cmd like this "winpmem_v3.3.rc3.exe -dd -o - --format raw --volume_format raw", it is not raw output, is affv4 format