fix: pass bytes.Reader to http.Request, clean up pooledReader ourselves

[lizthegrey]

Short description of the changes

• Passes bytes.Reader directly to http.Request to simplify logic and reduce likelihood of problems. Solves #158 and #154 more cleanly than #156 did.

[lizthegrey]

Leaving this for @MikeGoldsmith to pick up in his morning once he wakes up in about 12 hours!