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 <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: Trott

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);