Fixes a bug where `data` is not captured correctly.

[dom96]

Originally reported in: https://github.com/dom96/jester/issues/296. This took me longer to find than I'd like to admit. I did actually try closureScope first, but I think it didn't work because of the template that's here (even the capture has no effect on the contents of the template, which is why I changed it to take the data explicitly. This is quite a nasty gotcha in Nim).

Test plan:

nim c -r tests/delayed_writes.nim

wrk -c 10 -t 1 -d 10 http://localhost:5000/

Before:

Starting 1 threads
Listening on port 5000
SO_REUSEPORT not supported on this platform. HttpBeast will not utilise all threads.
/mnt/c/Users/morfe/projects/httpbeast/tests/delayed_writes.nim(22) delayed_writes
/mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(540) run
/mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(371) eventLoop
/mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(227) processEvents
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/pure/asyncdispatch.nim(1653) poll
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/pure/asyncdispatch.nim(1394) runOnce
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/pure/asyncdispatch.nim(234) processPendingCallbacks
/mnt/c/Users/morfe/projects/httpbeast/tests/delayed_writes.nim(15) :anonymous
/mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(456) send
/mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(421) send
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/system/assertions.nim(38) failedAssertImpl
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/system/assertions.nim(28) raiseAssert
/home/dom/.choosenim/toolchains/nim-1.6.6/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /mnt/c/Users/morfe/projects/httpbeast/src/httpbeast.nim(421, 12) `requestData.headersFinished` Selector not ready to send. [AssertionDefect]

After: no crash