switch MPD urls to protocol relative when available in https

Created by: jeffcunat

This PR allows some streams to be used when the client page is in https. At the moment, because of mixed content rule, many streams are not playable.

I have also tested the status (OK, NOK) of all test streams (sources.json and dashjs.json) with some remarks. It can be useful to clean up the list of streams. Some streams could simply be accessible in https and some manifests could be updated to use a in https too. I have noticed that many live streams are stil broken in Chrome because of this issue #93 (closed) (MseBufferByPts feature not enabled by default)

See the results in this excel file (two sheets, one for sources.json streams and one for dashjs test vectors) Test_Streams_Status.xlsx