child_process: fix sending utf-8 to child process

In process#send() and child_process.ChildProcess#send(), use 'utf8' as
the encoding and correctly handle partial character sequences by
introducing a StringDecoder. Before this commit, it used 'ascii' and
partial sequences were dropped or corrupted.

This is a back-port of commit 44843a6 from the v0.10 branch.

Fixes #4999 and #5011.