_debug_agent: use readableObjectMode option

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

Partially addresses #445.

PR-URL: #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>

lib/_debug_agent.js

@@ -83,8 +83,9 @@ Agent.prototype.notifyWait = function notifyWait() {
 };
 
 function Client(agent, socket) {
-  Transform.call(this);
-  this._readableState.objectMode = true;
+  Transform.call(this, {
+    readableObjectMode: true
+  });
 
   this.agent = agent;
   this.binding = this.agent.binding;