Fix updateHeaderExtension for cloned MediaEngine

[mission-liao]

Description

It addresses the issue that unable to updateHeaderExtension for cloned
MediaEngine, which is resulted from previous commit:

c8b7aa3

Reference issue

Fixes a possible(?) side-effect from the PR for #1662

[codecov]

Codecov Report

Merging #1682 (7c1f7aa) into master (19537f2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1682      +/-   ##
==========================================
- Coverage   75.02%   75.00%   -0.03%     
==========================================
  Files          79       79              
  Lines        5718     5720       +2     
==========================================
  Hits         4290     4290              
- Misses       1055     1056       +1     
- Partials      373      374       +1     

Flag	Coverage Δ
go	76.57% <100.00%> (-0.03%)	⬇️
wasm	69.90% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mediaengine.go	80.99% <100.00%> (+0.15%)	⬆️
dtlstransport.go	67.00% <0.00%> (-2.00%)	⬇️
peerconnection.go	73.84% <0.00%> (+0.17%)	⬆️

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[Sean-Der]

Nice catch! I think it’s ok to always initialize, not that much overhead vs complexity.

Tag a new version if you want, thanks so much :)

[mission-liao]

OK, would provide another commit to make it always initialize later. It would be less complicated.

[mission-liao]

I've looked around codes for negotiatedHeaderExtensions, and think it's still better to keep the last commit.

The behavior: updateHeaderExtension would do nothing if RegisterHeaderExtension is not called seems reasonable.

Being of two minds.