child_process: fix sending utf-8 to child process

In process#send() and child_process.ChildProcess#send(), use 'utf8' as
the encoding instead of 'ascii' because 'ascii' mutilates non-ASCII
input.

It worked by accident in v0.8 but not in v0.10 because the high bits
are now stripped when converting Buffers to ASCII strings. See commit
96a314b for details.

Fixes #4999 and #5011.