set the Content-Length for HTTP/3 responses

[marten-seemann]

Fixes #2398.

[codecov]

Codecov Report

Merging #3091 (29f02e1) into master (3bce408) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3091   +/-   ##
=======================================
  Coverage   85.72%   85.73%           
=======================================
  Files         131      131           
  Lines        9520     9529    +9     
=======================================
+ Hits         8161     8169    +8     
- Misses        990      991    +1     
  Partials      369      369           

Impacted Files	Coverage Δ
http3/client.go	89.35% <100.00%> (+0.60%)	⬆️
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

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