`EPOLLHUP` event causes `AssertionDefect` to be thrown

[shunf4]

https://github.com/xflywind/httpx/blob/7e8ca92efe921fbe2aa363ffafe3c443cc9b73fa/src/httpx.nim#L443-L444

When events[i].events contains neither Read nor Write, an AssertionDefect is thrown.

However, as per code lines in ioselectors:

https://github.com/xflywind/ioselectors/blob/6fcdbf3fddeb239885cd8bce15a0ab63c00647c0/src/ioselectors/ioselectors_wepoll.nim#L264-L270

only EPOLLOUT and EPOLLIN will be recorded in rkey.events. There is still the possibility for events like EPOLLHUP to take place, and cause rkey.events to be empty, and thus AssertionDefect is thrown in httpx:

...(25) ...
C:\Users\shunf4\.nimble\pkgs\prologue-0.4.4\prologue\core\application.nim(534) run
C:\Users\shunf4\.nimble\pkgs\prologue-0.4.4\prologue\core\beast\server.nim(31) serve
C:\Users\shunf4\.nimble\pkgs\httpx-0.2.4\httpx.nim(...) run
C:\Users\shunf4\.nimble\pkgs\httpx-0.2.4\httpx.nim(...) eventLoop
C:\Users\shunf4\.nimble\pkgs\httpx-0.2.4\httpx.nim(...) processEvents
C:\tools\Nim\nim-1.4.6\lib\system\assertions.nim(30) failedAssertImpl
C:\tools\Nim\nim-1.4.6\lib\system\assertions.nim(23) raiseAssert
C:\tools\Nim\nim-1.4.6\lib\system\fatal.nim(49) sysFatal
Error: unhandled exception: C:\Users\shunf4\.nimble\pkgs\httpx-0.2.4\httpx.nim(..., 16) `false`  [AssertionDefect]