http: Don't dump twice

lib/http.js

@@ -409,6 +409,9 @@ IncomingMessage.prototype._addHeaderLine = function(field, value) {
 // Call this instead of resume() if we want to just
 // dump all the data to /dev/null
 IncomingMessage.prototype._dump = function() {
+  if (this._dumped)
+    return;
+
   this._dumped = true;
   this.socket.parser.incoming = null;
   this.push(null);