Fix transcripts in XML course when no static_asset_path is set.

Case where static_asset_path is not set in XML course.

In such case use data_dir.
Issue: https://openedx.atlassian.net/browse/BLD-67

@cpennington @carsongee 
> I wonder whether it would be possible to make the LMS do the same rewriting of 30* Location headers that it does of links in html content, for instance. That way, all of this knowledge would move out to the runtime, and wouldn't be the responsibility of the video_module at all.

@cpennington Could you describe more or point to where rewriting happens please?