Test adding connection to Reactor after shutdown [changelog skip]

[wjordan]

Description

Quick followup to #2279, this PR extends the TestPumaServer#shutdown_requests test-method to pause Reactor#add for the second connection until after shutdown begins, to ensure this edge case is correctly handled.

This adds unit-test coverage for the fix introduced in #2377 and updated in #2279.

This added test revealed that the edge-case wasn't handled properly in Ruby 2.2, so that is fixed in commit 0f4e51f as well.

Finally, commit e5d04eb is a small style-fix followup in the related Reactor code.

Testing

Verified that this modified test fails on the following commits:

• de2f108 (commit before Reactor refactor #2279):

  1) Failure:
TestPumaServer#test_shutdown_requests [test/test_puma_server.rb:1016]:
Expected /204/ to match # encoding: ASCII-8BIT
#    valid: true
"HTTP/1.1 500 Internal Server Error\r\n".

  2) Failure:
TestPumaServer#test_force_shutdown [test/test_puma_server.rb:1018]:
Expected # encoding: ASCII-8BIT
#    valid: true
"HTTP/1.1 500 Internal Server Error\r\n" to be nil.

• e041d07 (commit before Prevent connections from entering Reactor after shutdown begins #2377):

Error reached top of thread-pool: closed stream (IOError)

  1) Failure:
TestPumaServer#test_shutdown_requests [test/test_puma_server.rb:1007]:
timeout waiting for response

  2) Failure:
TestPumaServer#test_force_shutdown [test/test_puma_server.rb:1007]:
timeout waiting for response

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