net: replace _writableState.finished with writableFinished

Replace usage of quasi-private _writableState.finished with public
writableFinished property.

PR-URL: #27974
Refs: #445
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/net.js

@@ -556,7 +556,7 @@ Socket.prototype.destroySoon = function() {
   if (this.writable)
     this.end();
 
-  if (this._writableState.finished)
+  if (this.writableFinished)
     this.destroy();
   else
     this.once('finish', this.destroy);