Running a simple example based on the `httpbeast 0.2.2` framework made an error.
Running a simple example based on the `httpbeast 0.2.2` framework made an error :`getData.headersFinished` Selector not ready to send.[AssertionDefect].

Note: When i use browser request, there is no error, but when i use wrk -c100-d20--latency http://192.12.1.1:8080/ for stress test, there is an error.

My code is as follows:

```nim
import segfaults
import random

import options, asyncdispatch

import httpbeast

proc onRequest(req: Request): Future[void] {.async.} =
  if req.httpMethod == some(HttpGet):
    case req.path.get()
    of "/":
      randomize()
      let t = rand(999)
      await sleepAsync(t)
      req.send("Hello World")
    else:
      req.send(Http404)

run(onRequest)
```

The errors I have obtained are as follows:
```
Starting 6 threads
Listening on port 8080
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(287) eventLoop
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(166) processEvents
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(1626) poll
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(1367) runOnce
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
/home/lhf/Nim-devel/lib/pure/asyncmacro.nim(22) onRequestNimAsyncContinue
/home/lhf/桌面/t.nim(15) onRequestIter
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(342) send
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(321) send
/home/lhf/Nim-devel/lib/system/assertions.nim(30) failedAssertImpl
/home/lhf/Nim-devel/lib/system/assertions.nim(23) raiseAssert
/home/lhf/Nim-devel/lib/system/fatal.nim(49) sysFatal
[[reraised from:
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(287) eventLoop
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(166) processEvents
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(1626) poll
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(1367) runOnce
/home/lhf/Nim-devel/lib/pure/asyncdispatch.nim(208) processPendingCallbacks
/home/lhf/Nim-devel/lib/pure/asyncfutures.nim(281) :anonymous
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(223) :anonymous
/home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(94) onRequestFutureComplete
]]
Error: unhandled exception: /home/lhf/.nimble/pkgs/httpbeast-0.2.2/httpbeast.nim(321, 10) `getData.headersFinished` Selector not ready to send. [AssertionDefect]
```
