Correctly decode UTF8 fork messages.

Messages to child processes are stringified, sent as Buffers,
converted to strings and then parsed. The process of converting the
message to a string was incorrectly using the 'ascii' encoding, which
causes the JSON to become unparsable.

This was introduced by new restrictions on encoding conversion. In
previous versions of Node, attempting to convert a UTF8 Buffer to a
string using the 'ascii' encoding was allowed. That is no longer the
case however.