Uniformly treat zero-length bodies as missing

[vkostyukov]

Fixes #795.

[codecov-io]

Codecov Report

Merging #796 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #796     +/-   ##
=========================================
+ Coverage    79.7%   79.91%   +0.2%     
=========================================
  Files          44       44             
  Lines         680      682      +2     
  Branches       30       30             
=========================================
+ Hits          542      545      +3     
+ Misses        138      137      -1

Impacted Files	Coverage Δ
core/src/main/scala/io/finch/endpoint/body.scala	61.76% <100%> (+5.51%)	⬆️
core/src/main/scala/io/finch/Endpoint.scala	73.07% <0%> (-0.77%)	⬇️
core/src/main/scala/io/finch/internal/Rs.scala	93.75% <0%> (+6.25%)	⬆️

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