Req.normalize_headers

You're seeing just the function normalize_headers, go back to Req module for more information.
Link to this function

normalize_headers(request)

View Source

Normalizes request headers.

Turns atom header names into strings, e.g.: :user_agent becomes "user-agent". Non-atom names are returned as is.

Examples

iex> Req.get!("https://httpbin.org/user-agent", headers: [user_agent: "my_agent"]).body
%{"user-agent" => "my_agent"}