dash: allow range in SegmentList's Initialization

I have a single-file segment list which looks like this:

    <SegmentList duration="2020" timescale="1000">
      <Initialization range="28-619" sourceURL="audio.mp4"/>
      <SegmentURL media="audio.mp4" mediaRange="620-31006"/>
       ...
    </SegmentList>

dash.js did not take account of the ``range`` attribute in the ``Initialization`` element and downloaded the whole video instead.

Here is a small fix for this issue.