_debug_agent: use `readableObjectMode` option

Use public `readableObjectMode` option to construct `Transform`
instead of accessing private `_readableState.objectMode`.

Partially addresses https://github.com/iojs/io.js/issues/445.

PR-URL: https://github.com/iojs/io.js/pull/270


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>