[Manifest] builds contents as bytes

The [Manifest] plugin builds content for the MANIFEST file
from code, but doesn't declare that that code returns bytes,
rather than characters. This means that if you have
"interesting" files in your dist (which may or may not be a
bad idea) then you'll get warnings about missing files:

WARNING: the following files are missing in your kit:
    corpus/ã¿ãã¡ãã

because the manifest check is expecting the garbage filename,
but your dist actually includes the file with the correct
filename. No file is missing, but `MANIFEST` is wrong.

This simply adds the appropriate declaration when creating
the MANIFEST file.