streams2: Set 'readable' flag on Readable streams

lib/_stream_readable.js

@@ -99,6 +99,10 @@ function Readable(options) {
     return new Readable(options);
 
   this._readableState = new ReadableState(options, this);
+
+  // legacy
+  this.readable = true;
+
   Stream.apply(this);
 }