Add #string method to Puma::NullIO

[dentarg]

Description

Makes Puma::NullIO more similar to StringIO.

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• I have reviewed the guidelines for contributing to this repository.
• I have added an entry to History.md if this PR fixes a bug or adds a feature. If it doesn't need an entry to HISTORY.md, I have added [changelog skip] or [ci skip] to the pull request title.
• I have added appropriate tests if this PR fixes a bug or adds a feature.
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• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[dentarg]

CI failure looks unrelated, testing a re-run (of all jobs sadly), the failure was:

 48) Failure:
TestIntegrationSingle#test_hot_restart_does_not_drop_connections [D:/a/puma/puma/test/helpers/integration.rb:304]:
      0 unexpected_response
      5 refused
      0 read timeout
     13 reset
    482 success
    468 success after restart
     13 restart count
Expected no refused connections.
Expected: 0
  Actual: 5

407 runs, 994 assertions, 1 failures, 0 errors, 47 skips

------------------------------------------------------------ Debugging Info
TestIntegrationSingle#test_hot_restart_does_not_drop_connections_threads
   restart_count 4, reset 20, success after restart 901
TestIntegrationSingle#test_hot_restart_does_not_drop_connections
   restart_count 13, reset 13, success after restart 467
TestIntegrationSingle#test_hot_restart_does_not_drop_connections
   restart_count 11, reset 11, success after restart 474
TestIntegrationSingle#test_hot_restart_does_not_drop_connections
   restart_count 12, reset 12, success after restart 470
TestIntegrationSingle#test_hot_restart_does_not_drop_connections
   restart_count 13, reset 13, success after restart 468
---------------------------------------------------------------------------

[dentarg]

**MRI / windows-2019 2.7 (pull_request) ** Successful in 6m

Yay (this was the failing job, didn't make that clear in my earlier comment)

[dentarg]

MRI / macos-11.0 2.7
This check failed

Thanks GitHub Actions, that's helpful 🤷