Support Stripe-Should-Retry header

As seen in stripe-node, adds support for the `Stripe-Should-Retry`
header which is sent by the API when it explicitly would like us to
either retry or _not_ retry.

I'll add `Retry-After` separately at some point, but I punted it on it
for now given that we're not using it yet.

See: stripe/stripe-node#692

Author: brandur

stripe.go

@@ -576,6 +576,16 @@ func (s *BackendImplementation) shouldRetry(err error, req *http.Request, resp *
 		return true
 	}
 
+	// The API may ask us not to retry (e.g. if doing so would be a no-op), or
+	// advise us to retry (e.g. in cases of lock timeouts). Defer to those
+	// instructions if given.
+	if resp.Header.Get("Stripe-Should-Retry") == "false" {
+		return false
+	}
+	if resp.Header.Get("Stripe-Should-Retry") == "true" {
+		return true
+	}
+
 	// 409 Conflict
 	if resp.StatusCode == http.StatusConflict {
 		return true

stripe_test.go

@@ -156,6 +156,35 @@ func TestShouldRetry(t *testing.T) {
 		0,
 	))
 
+	// `Stripe-Should-Retry: false`
+	assert.False(t, c.shouldRetry(
+		nil,
+		&http.Request{},
+		&http.Response{
+			Header: http.Header(map[string][]string{
+				"Stripe-Should-Retry": {"false"},
+			}),
+			// Note we send status 409 here, which would normally be retried
+			StatusCode: http.StatusConflict,
+		},
+		0,
+	))
+
+	// `Stripe-Should-Retry: true`
+	assert.True(t, c.shouldRetry(
+		nil,
+		&http.Request{},
+		&http.Response{
+			Header: http.Header(map[string][]string{
+				"Stripe-Should-Retry": {"true"},
+			}),
+			// Note we send status 400 here, which would normally not be
+			// retried
+			StatusCode: http.StatusBadRequest,
+		},
+		0,
+	))
+
 	// 409 Conflict
 	assert.True(t, c.shouldRetry(
 		nil,