fix HTTP request writing if the Request.Body reads data and returns EOF

[marten-seemann]

Fixes #2641.

[codecov]

Codecov Report

Merging #2642 into master will decrease coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2642      +/-   ##
==========================================
- Coverage   86.37%   86.36%   -0.01%     
==========================================
  Files         120      120              
  Lines        9743     9748       +5     
==========================================
+ Hits         8415     8418       +3     
- Misses        992      993       +1     
- Partials      336      337       +1     

Impacted Files	Coverage Δ
http3/request_writer.go	65.13% <83.33%> (-0.17%)	⬇️

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