Add StripeError#idempotent_replayed? (#907)

Adds an easy accessor on `StripeError` which indicates whether the error
occurred previously, but was replayed on this request because the user
passed the same idempotency key as that original request.

Fixes #905.

lib/stripe/errors.rb

@@ -25,6 +25,7 @@ def initialize(message = nil, http_status: nil, http_body: nil,
       @http_status = http_status
       @http_body = http_body
       @http_headers = http_headers || {}
+      @idempotent_replayed = @http_headers["idempotent-replayed"] == "true"
       @json_body = json_body
       @code = code
       @request_id = @http_headers["request-id"]
@@ -37,6 +38,13 @@ def construct_error_object
       ErrorObject.construct_from(@json_body[:error])
     end
 
+    # Whether the error was the result of an idempotent replay, meaning that it
+    # originally occurred on a previous request and is being replayed back
+    # because the user sent the same idempotency key for this one.
+    def idempotent_replayed?
+      @idempotent_replayed
+    end
+
     def to_s
       status_string = @http_status.nil? ? "" : "(Status #{@http_status}) "
       id_string = @request_id.nil? ? "" : "(Request #{@request_id}) "

test/stripe/errors_test.rb

@@ -14,6 +14,18 @@ class StripeErrorTest < Test::Unit::TestCase
         end
       end
 
+      context "#idempotent_replayed?" do
+        should "initialize from header" do
+          e = StripeError.new("message", http_headers: { "idempotent-replayed" => "true" })
+          assert_equal true, e.idempotent_replayed?
+        end
+
+        should "be 'falsey' by default" do
+          e = StripeError.new("message")
+          refute_equal true, e.idempotent_replayed?
+        end
+      end
+
       context "#to_s" do
         should "convert to string" do
           e = StripeError.new("message")