ebuild: fix encoding of send_env

Please review, I find encodings to be a pretty huge mess and thus am not sure,
that this is the correct solution. However it makes things work on my end.

_generate_env_str returned a unicode string in py3 which needs to be encodod
for serialization. This encodes the string if necessary. Note that ret prior
to encoding may either be bytes or unicode in py2 (it is unicode if data
contains a unicode string).

Rename _generate_env_str to reflect this change.