reqwest::headers

[AndyGauge]

We ignored and filtered the headers recipe, but now reqwest headers work.

https://github.com/rust-lang-nursery/rust-cookbook/blob/master/src/web/clients/requests/header.md

https://stackoverflow.com/a/47912021/5586783 https://docs.rs/reqwest/0.10.6/reqwest/header/index.html