pad datagrams containing ack-eliciting Initial packets sent by the server

[marten-seemann]

Fixes #2840.

[codecov]

Codecov Report

Merging #2841 into master will decrease coverage by 0.02%.
The diff coverage is 86.67%.

@@            Coverage Diff             @@
##           master    #2841      +/-   ##
==========================================
- Coverage   85.83%   85.80%   -0.02%     
==========================================
  Files         134      134              
  Lines        9030     9036       +6     
==========================================
+ Hits         7750     7753       +3     
- Misses        939      941       +2     
- Partials      341      342       +1     

Impacted Files	Coverage Δ
packet_packer.go	83.49% <83.33%> (-0.53%)	⬇️
internal/ackhandler/ack_eliciting.go	100.00% <100.00%> (ø)

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