http3: ignore context after response when using DontCloseRequestStream

[marten-seemann]

In WebTransport, the request stream is kept alive for the lifetime of the WebTransport session (and used to communicate shutdown of that session).

This leads to problems when using a ctx in Roundtrip. The following code should be valid, using ctx to control cancelation of the WebTransport handshake:

ctx, cancel := context.WithCancel(context.Background())
req = req.WithContext(ctx)
rsp, _ := rountripper.RoundTrip(req)
cancel()

However, it also resets the request stream (and thereby closes the WebTransport session) as soon as the WebTransport handshake completes.

[codecov]

Codecov Report

Merging #3473 (418532e) into master (f29dd27) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3473      +/-   ##
==========================================
+ Coverage   85.66%   85.68%   +0.01%     
==========================================
  Files         137      137              
  Lines       10009    10019      +10     
==========================================
+ Hits         8574     8584      +10     
  Misses       1059     1059              
  Partials      376      376              

Impacted Files	Coverage Δ
http3/roundtrip.go	93.55% <ø> (ø)
http3/body.go	78.57% <100.00%> (+0.79%)	⬆️
http3/client.go	90.41% <100.00%> (+0.41%)	⬆️

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