add support for idempotent post requests (beam-community#461)

lib/stripe.ex

@@ -29,6 +29,8 @@ defmodule Stripe do
       [https://stripe.com/docs/connect/authentication#stripe-account-header](https://stripe.com/docs/connect/authentication#stripe-account-header)
     * `:expand` - Takes a list of fields that should be expanded in the response
       from Stripe. See [https://stripe.com/docs/api/expanding_objects](https://stripe.com/docs/api/expanding_objects)
+    * `:idempotency_key` - A string that is passed through as the "Idempotency-Key" header on all POST requests. This is used by Stripe's idempotency layer to manage
+      duplicate requests to the stripe API. See [https://stripe.com/docs/api/idempotent_requests](https://stripe.com/docs/api/idempotent_requests)
 
   ### HTTP Connection Pool
 

lib/stripe/api.ex

@@ -165,9 +165,11 @@ defmodule Stripe.API do
 
   def request(body, method, endpoint, headers, opts) do
     {expansion, opts} = Keyword.pop(opts, :expand)
-    base_url = get_base_url()
+    {idempotency_key, opts} = Keyword.pop(opts, :idempotency_key)
 
+    base_url = get_base_url()
     req_url = add_object_expansion("#{base_url}#{endpoint}", expansion)
+    headers = add_idempotency_header(idempotency_key, headers, method)
 
     req_body =
       body
@@ -317,4 +319,12 @@ defmodule Stripe.API do
   end
 
   defp add_object_expansion(url, _), do: url
+
+  defp add_idempotency_header(nil, headers, _), do: headers
+
+  defp add_idempotency_header(idempotency_key, headers, :post) do
+    Map.put(headers, "Idempotency-Key", idempotency_key)
+  end
+
+  defp add_idempotency_header(_, headers, _), do: headers
 end

test/stripe/core_resources/charge_test.exs

@@ -30,6 +30,13 @@ defmodule Stripe.ChargeTest do
     assert_stripe_requested(:post, "/v1/charges/ch_123/capture")
   end
 
+  test "is captureable with idempotency opts" do
+    opts = [idempotency_key: "test"]
+    {:ok, %Stripe.Charge{} = charge} = Stripe.Charge.retrieve("ch_123")
+    assert {:ok, %Stripe.Charge{}} = Stripe.Charge.capture(charge, %{amount: 1000}, opts)
+    assert_stripe_requested(:post, "/v1/charges/ch_123/capture")
+  end
+
   test "is retrievable with expansions opts" do
     opts = [expand: ["balance_transaction"]]
     assert {:ok, %Stripe.Charge{}} = Stripe.Charge.retrieve("ch_123", opts)