Removes the httpuv connection: close workaround

[atheriel]

This removes the workaround introduced in 4901c5e to avoid rstudio/httpuv#49, which has been fixed (in my estimation) since at least httpuv 1.4.0.

I've tentatively confirmed that environments are no longer shared between connections even when they are keep-alive by using the original integration test added in the referenced commit.

As a nice bonus, this improves plumber's baseline latency by about 10% on my machine.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #318 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #318      +/-   ##
========================================
- Coverage   90.01%    90%   -0.01%     
========================================
  Files          25     25              
  Lines        1142   1141       -1     
========================================
- Hits         1028   1027       -1     
  Misses        114    114

Impacted Files	Coverage Δ
R/response.R	100% <ø> (ø)	⬆️
R/plumber.R	84.74% <ø> (ø)	⬆️

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[wch]

Looks good to me.

[trestletech]

There's one more file in the commit you referenced: 4901c5e#diff-68678c8c847a2f72255e66a25514f0abR34 should we also update that one?

[schloerke]

@atheriel Thank you for the change! Could you please sign the CLA for when I merge the PR next week? Link: https://cla-assistant.io/trestletech/plumber?pullRequest=318

[atheriel]

@schloerke Ah, I didn't realize I'd need to sign for this change, since I did not contribute any copyrightable material or code. It should show as signed now.