Refactor RequestBodyReader

[jwoertink]

Purpose

Fixes #1825

Description

The original implementation of this was a bit difficult to read and understand. This is just a refactor to make the intention of the method a little easier to comprehend. Added some docs.

Also ran some benchmarks and this way is slightly faster:

old  11.59k ( 86.27µs) (± 1.24%)  578kB/op   1.45× slower
new  16.75k ( 59.70µs) (± 0.75%)  406kB/op        fastest

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test