add fix for multiple set-cookies API Gateway limitation

relevant forum post about the limitation:
https://forums.aws.amazon.com/thread.jspa?threadID=205782&start=25&tstart=0

http/relay/relay.go

@@ -174,6 +174,12 @@ retry:
 	r.URL.Host = p.target.Host
 	res, err := DefaultTransport.RoundTrip(r)
 
+	// API Gateway does not support multiple set-cookie fields
+	// so we have to stagger the casing in order to support this.
+	if err == nil {
+		util.FixMultipleSetCookie(res.Header)
+	}
+
 	// retries disabled, don't create noise in the logs
 	if p.maxRetries == 0 {
 		return res, err

internal/util/util.go

@@ -490,3 +490,44 @@ func StripLerna(s string) string {
 
 	return s
 }
+
+// FixMultipleSetCookie staggers the casing of each set-cookie
+// value to trick API Gateway into setting multiple in the response.
+func FixMultipleSetCookie(h http.Header) {
+	cookies := h["Set-Cookie"]
+
+	if len(cookies) == 0 {
+		return
+	}
+
+	h.Del("Set-Cookie")
+
+	for i, v := range cookies {
+		h[BinaryCase("set-cookie", i)] = []string{v}
+	}
+}
+
+// BinaryCase ported from https://github.com/Gi60s/binary-case/blob/master/index.js#L86.
+func BinaryCase(s string, n int) string {
+	var res []rune
+
+	for _, c := range s {
+		if c >= 65 && c <= 90 {
+			if n&1 > 0 {
+				c += 32
+			}
+			res = append(res, c)
+			n >>= 1
+		} else if c >= 97 && c <= 122 {
+			if n&1 > 0 {
+				c -= 32
+			}
+			res = append(res, c)
+			n >>= 1
+		} else {
+			res = append(res, c)
+		}
+	}
+
+	return string(res)
+}

internal/util/util_test.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"net/http"
 	"os/exec"
 	"strings"
 	"testing"
@@ -123,3 +124,38 @@ func TestEncodeAlias(t *testing.T) {
 func TestDecodeAlias(t *testing.T) {
 	assert.Equal(t, `v1.2.3-beta`, DecodeAlias(EncodeAlias(`v1.2.3-beta`)))
 }
+
+func TestFixMultipleSetCookie(t *testing.T) {
+	h := http.Header{}
+	h.Add("Set-Cookie", "first=tj")
+	h.Add("Set-Cookie", "last=holowaychuk")
+	h.Add("set-cookie", "pet=tobi")
+	FixMultipleSetCookie(h)
+	assert.Len(t, h, 3)
+	assert.Equal(t, []string{"last=holowaychuk"}, h["Set-cookie"])
+	assert.Equal(t, []string{"pet=tobi"}, h["sEt-cookie"])
+	assert.Equal(t, []string{"first=tj"}, h["set-cookie"])
+}
+
+func TestBinaryCase(t *testing.T) {
+	var variations []string
+
+	// create variations
+	for i := 0; i < 50; i++ {
+		variations = append(variations, BinaryCase("set-cookie", i))
+	}
+
+	// ensure none are malformed
+	for _, v := range variations {
+		assert.Equal(t, "set-cookie", strings.ToLower(v))
+	}
+
+	// ensure none are duplicates
+	for i, a := range variations {
+		for j, b := range variations {
+			if i != j {
+				assert.NotEqual(t, a, b)
+			}
+		}
+	}
+}