add http "header" support to $media.play

I don't see in JasonMediaAction.m any use of @"headers" which explains why it doesn't work when I add headers in my markup. It would be great to be able to use "header" (or better yet, "http_headers") anywhere that "url" is valid.

```
"action":{
"type":"$media.play",
"options":{
"url":"{{video['mp4']}}",
"header":{
"referer":"http://whatever.com/"
}
}
}
```

possibly useful: [stackoverflow](https://stackoverflow.com/questions/15456130/send-headers-with-avplayer-request-in-ios)

Start in JasonMediaAction.m and trace how the $media.play options consume the url. Review the linked AVPlayer header-request reference, then verify that the requested headers are accepted with the media URL and that playback succeeds using the example markup.

Written by the indexing model from the issue text.