net: use decodeStrings public API for writable stream

Instead of using an undocumented underscore-prefixed property to
configure the writable stream instance to not encode strings as buffers,
use the public API which is an options property passed to the
constructor.

Refs: nodejs#445

PR-URL: nodejs#25201
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/net.js

@@ -255,6 +255,8 @@ function Socket(options) {
   options.allowHalfOpen = true;
   // For backwards compat do not emit close on destroy.
   options.emitClose = false;
+  // Handle strings directly.
+  options.decodeStrings = false;
   stream.Duplex.call(this, options);
 
   // Default to *not* allowing half open sockets.
@@ -308,9 +310,6 @@ function Socket(options) {
   this._pendingData = null;
   this._pendingEncoding = '';
 
-  // handle strings directly
-  this._writableState.decodeStrings = false;
-
   // If we have a handle, then start the flow of data into the
   // buffer.  if not, then this will happen when we connect
   if (this._handle && options.readable !== false) {