Fix #1321 - relative MPD@BaseURL are not resolved correctly

Created by: davemevans

See #1321 (closed) for discussion. Also added some tests for the same issue and others.

Also noticed that http://dash.edgesuite.net/akamai/test/caption_test/ElephantsDream/elephants_dream_480p_heaac5_1.mpd no longer played back correctly. I'm not convinced the signalling in the manifest is legal, but it used to work so made it work again.

I would recommend this gets pulled in for 2.1.0.