Fix: retain playlist attributes when refreshing live media playlists

[mister-ben]

Description

On a live playlist with media groups, subsequent fetches of the media playlist overwrite the playlist attributes with an empty object. This affects part of the rendiiton selection flow, including the audio only test. On an audio-only stream this impacts playback, as no audio only stream is found. This can result in cycling through the renditions, playing an unnecessarily low rendtion, or for streams with manifest redundancy, that playback stops.

Specific Changes proposed

Updates th playlist with the merged playlist rather than the unmodifed newly parsed playlist. This should have been the behaviour already.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #1270 (10edac7) into main (b98877d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1270   +/-   ##
=======================================
  Coverage   86.30%   86.30%           
=======================================
  Files          39       39           
  Lines        9840     9840           
  Branches     2292     2292           
=======================================
  Hits         8492     8492           
  Misses       1348     1348           

Impacted Files	Coverage Δ
src/playlist-loader.js	95.04% <100.00%> (ø)

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