Crash when refreshing too quickly
Perhaps this issue should be at httpbeast's repo, but I encountered it when using Jester.

If the end user refresh a page quickly multiple times the program crash with:
```nim
main.nim(1193)           main  # This is where `routes:` start
jester.nim(381)          serve
httpbeast.nim(388)       run
httpbeast.nim(267)       eventLoop
httpbeast.nim(160)       processEvents
system.nim(3810)         failedAssertImpl
system.nim(3803)         raiseAssert
system.nim(2857)         sysFatal
Error: unhandled exception: len(data.sendQueue) == 0  [AssertionError]
```

There is no consistency in the last GET:
1) `DEBUG   200 OK {"etag": @["5079718ceac00853f7de844fe0bb31aa"], "content-length": @["10797"], "content-type": @["application/javascript"]}`
2) `DEBUG   200 OK {"content-length": @["77444"], "content-type": @["text/html;charset=utf-8"]}`
3) `DEBUG   200 OK {"etag": @["d2f052efc0afb5607a6802f4dc35e130"], "content-length": @["28231"], "content-type": @["image/png"]}`

Nim Compiler Version 0.18.1 [Linux: amd64]
Jester@#081e5bd82e6eedeff8646e4259bd94996f3478e3
