Proposal: Expose headers to class functions for Connect usage

[dmvt]

When acting on behalf of a Connected account, Stripe recommends using the Stripe-Account header as seen here: https://stripe.com/docs/connect/payments-fees#charging-directly

I propose updating the class functions to expose headers in the following way:

This:

  def create(params, key) do
    Stripe.make_request_with_key(:post, @endpoint, key, params)
    |> Stripe.Util.handle_stripe_response
  end

would become this:

  def create(params, key_or_headers) when is_bitstring(key_or_headers) do
    Stripe.make_request_with_key(:post, @endpoint, key_or_headers, params)
    |> Stripe.Util.handle_stripe_response
  end

  def create(params, key_or_headers) when is_map(key_or_headers) do
    Stripe.make_request_with_key(
      :post, @endpoint, Stripe.config_or_env_key, params, key_or_headers
    )
    |> Stripe.Util.handle_stripe_response
  end

This still allows a key override but also supports passing any headers required without using the low level API on the Stripe class. With your approval, I'll submit a PR in the next week or so.