fix closing of the http.Request.Body

[marten-seemann]

Fixes #2551.

On the server side, the http.Request is consumed by the HTTP handler. The HTTP handler may close the body (it doesn't have to though). In any case, closing the stream is the wrong thing to do, since that closes the write side of the stream. All we want to do is cancel the stream (if the
EOF hasn't been read yet).

On the client side (for the http.Response) the same logic applies: When the application calls Body.Close(), all we want to do is cancel the stream (if the EOF hasn't been read yet).

[codecov]

Codecov Report

Merging #2584 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2584      +/-   ##
==========================================
- Coverage   86.28%   86.27%   -0.01%     
==========================================
  Files         122      122              
  Lines        9744     9738       -6     
==========================================
- Hits         8407     8401       -6     
  Misses        996      996              
  Partials      341      341              

Impacted Files	Coverage Δ
http3/body.go	100.00% <100.00%> (ø)
http3/server.go	64.63% <100.00%> (-0.46%)	⬇️

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[mholt]

Ah, you figured it out!! That's amazing! Would you like me to test as well? Let me know if I can do anything to help!