Prologue server crashes with unhandled exception: `reqGetData.headersFinished` Selector not ready to send.
Running prologue in single threaded mode (on macOS), server crashes on page refresh. Looks like it's happening while attempting to serve static css file with message: `DEBUG file: 31 is closed!`

Noticed Mostik mentioned this same issue in Discord channel on 11/21/2020:

> Hi, I worked in Linux and when I pressed in browser Ctrl + F5 all work, page update, but now I run my project on Windows and when I pressed Ctrl + F5 app crashes with error: .nimble\pkgs\httpx-0.2.4\httpx.nim(146, 10) reqGetData.headersFinished Selector not ready to send.

Debug log below:
```sh
DEBUG file: 31 is closed!
DEBUG file: 31 is closed!
/Users/op/code/apptest/src/apptest.nim(34) apptest
/Users/op/nim/prologue/src/prologue/core/application.nim(534) run
/Users/op/nim/prologue/src/prologue/core/beast/server.nim(31) serve
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(549) run
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(438) eventLoop
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(294) processEvents
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(1627) poll
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(1368) runOnce
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncmacro.nim(29) handleContextNimAsyncContinue
/Users/op/nim/prologue/src/prologue/core/application.nim(489) handleContextIter
/Users/op/nim/prologue/src/prologue/core/context.nim(276) respond
/Users/op/nim/prologue/src/prologue/core/beast/request.nim(147) respond
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(172) respond
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(146) send
/usr/local/Cellar/nim/1.4.2/nim/lib/system/assertions.nim(30) failedAssertImpl
/usr/local/Cellar/nim/1.4.2/nim/lib/system/assertions.nim(23) raiseAssert
/usr/local/Cellar/nim/1.4.2/nim/lib/system/fatal.nim(49) sysFatal
[[reraised from:
/Users/op/code/apptest/src/apptest.nim(34) apptest
/Users/op/nim/prologue/src/prologue/core/application.nim(534) run
/Users/op/nim/prologue/src/prologue/core/beast/server.nim(31) serve
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(549) run
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(438) eventLoop
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(294) processEvents
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(1627) poll
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(1368) runOnce
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
/usr/local/Cellar/nim/1.4.2/nim/lib/pure/asyncfutures.nim(281) :anonymous
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(359) :anonymous
/Users/op/.nimble/pkgs/httpx-0.2.4/httpx.nim(217) onRequestFutureComplete
]]
```