buffer up to 2x pre buffer size

lib/reader.js

@@ -118,7 +118,7 @@ function HlsStreamReader(src, options) {
 
   if (this.prebufferSize) {
     var lwm = options.lowWaterMark || 0;
-    var hwm = options.highWaterMark || this.prebufferSize;
+    var hwm = options.highWaterMark || this.prebufferSize * 2;
     options.lowWaterMark = Math.max(this.prebufferSize, lwm);
     options.highWaterMark = Math.max(hwm, lwm);
     this.once('readable', function() {