Return error for IoRef::write(), IoRef::with_write_buf(), Io::poll_flush() if io is closed

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.11%. Comparing base (effce69) to head (21f84da). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ntex-io/src/ioref.rs	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #432 +/- ## ========================================== - Coverage 91.12% 91.11% -0.02% ========================================== Files 200 200 Lines 32159 32161 +2 ========================================== - Hits 29305 29303 -2 - Misses 2854 2858 +4 ```

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