Invalid headers format using WebSocket

[Neylix]

Hello there ! Firstly I would thanks you for this library.

While the querying part over HTTP works well, I encountered an issue using WebSocket. When I try to connect an absinthe web socket I get an error from Req even before sending the request. The code triggering the error:

Req.new()
|> AbsintheClient.attach(graphql: "query { #{format_graphql(body)} }")
|> Req.merge(method: :post, url: "/api")
|> Req.request()

The interesting part of the error is :

  {:error,
   %NimbleOptions.ValidationError{
     message:
       ~s(invalid value for :headers option: expected list, got: %{"accept-encoding" => ["gzip"], "user-agent" => ["req/0.5.4"]}),
     key: :headers,
     value: %{"accept-encoding" => ["gzip"], "user-agent" => ["req/0.5.4"]},
     keys_path: []
   }}

The error happens on https://github.com/CargoSense/absinthe_client/blob/c178e4d2daa74e00d9773997d4e7e9bc47d1f02e/lib/absinthe_client/web_socket.ex#L173

From the documentation of SlipStream on version 1.1.1 "Headers must be provided as two-tuples where both elements are binaries" while the Req headers format is a map.

I locally fixed the issue replacing req.headers by

Enum.map(req.headers, fn {key, value} -> {key, Jason.encode!(value)} end)

But I'm not sure using Jason to encode values as string is the best option.