net: fix Socket({ fd: 42 }) api

Make the implementation match the documentation. This should work:

  var s = new net.Socket({ fd: 42, allowHalfOpen: true };

And now it does.

lib/net.js

@@ -131,20 +131,26 @@ function Socket(options) {
 
   Stream.call(this);
 
-  if (typeof options == 'number') {
-    // Legacy interface.
-    var fd = options;
+  switch (typeof options) {
+  case 'number':
+    options = { fd: options }; // Legacy interface.
+    break;
+  case 'undefined':
+    options = {};
+    break;
+  }
+
+  if (typeof options.fd === 'undefined') {
+    this._handle = options && options.handle; // private
+  } else {
     this._handle = createPipe();
-    this._handle.open(fd);
+    this._handle.open(options.fd);
     this.readable = this.writable = true;
-    initSocketHandle(this);
-  } else {
-    // private
-    this._handle = options && options.handle;
+  }
+
   initSocketHandle(this);
   this.allowHalfOpen = options && options.allowHalfOpen;
 }
-}
 util.inherits(Socket, Stream);