Don't append segments that have already been appended

Fixes #430. If drainBuffer is called after updating is reset on the sourceBuffer but before updateend fires, a segment could be appended twice. Check to make sure a segment hasn't already been appended before proceeding with draining the buffer.

@@ -948,6 +948,12 @@ videojs.Hls.prototype.drainBuffer = function(event) {
     return;
   }
 
+  // the pending segment has already been appended and we're waiting
+  // for updateend to fire
+  if (this.pendingSegment_.buffered) {
+    return;
+  }
+
   // we can't append more data if the source buffer is busy processing
   // what we've already sent
   if (this.sourceBuffer.updating) {

@@ -1180,16 +1180,24 @@ test('only makes one segment request at a time', function() {
 });
 
 test('only appends one segment at a time', function() {
+  var appends = 0;
   player.src({
     src: 'manifest/media.m3u8',
     type: 'application/vnd.apple.mpegurl'
   });
   openMediaSource(player);
   standardXHRResponse(requests.pop()); // media.m3u8
+  player.tech_.hls.sourceBuffer.appendBuffer = function() {
+    appends++;
+  };
+
   standardXHRResponse(requests.pop()); // segment 0
 
   player.tech_.hls.checkBuffer_();
   equal(requests.length, 0, 'did not request while updating');
+
+  player.tech_.hls.checkBuffer_();
+  equal(appends, 1, 'appended once');
 });
 
 test('waits to download new segments until the media playlist is stable', function() {