child_process: properly support optional args

Currently, a TypeError is incorrectly thrown if the second argument is
an object. This commit allows the args argument to be properly omitted.

Fixes: https://github.com/joyent/node/issues/6068
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Merge remote-tracking branch 'upstream/v0.10' into v0.10