Don't forward user's RTP header extensions in send_rtp/4

[mickel8]

This PR ensures that we don't accidentally forward non-negotiated RTP header extensions in RTP packet.

When playing with simulcast, it is very common that send_rtp/4 receives an RTP packet that contains extensions related to simulcast (e.g. RID). We should not forward them to the other side if they were not negotiated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.08%. Comparing base (d74a937) to head (1a25130).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   88.03%   88.08%   +0.05%     
==========================================
  Files          47       47              
  Lines        2381     2384       +3     
==========================================
+ Hits         2096     2100       +4     
+ Misses        285      284       -1     

Files with missing lines	Coverage Δ
lib/ex_webrtc/peer_connection.ex	85.94% <100.00%> (+0.20%)	⬆️
lib/ex_webrtc/peer_connection/configuration.ex	94.89% <ø> (ø)

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