fix flaky cancelation integration test

[marten-seemann]

When a send stream is canceled (either by the user calling CancelWrite(), or by receiving a STOP_SENDING frame), the Close() method will return an error. This happened occasionally in the cancellation integration test.

On a more general note, I'm not sure if this API makes sense. OTOH, this is the only way for the user to learn that a stream has been canceled if the STOP_SENDING frame is received after the Write() call returned. We should probably include the error code in the error message then. @lucas-clemente, wdyt?

[codecov]

Codecov Report

Merging #2649 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2649   +/-   ##
=======================================
  Coverage   86.37%   86.37%           
=======================================
  Files         122      122           
  Lines        9757     9757           
=======================================
  Hits         8427     8427           
  Misses        995      995           
  Partials      335      335           

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