google: Changes required to get AWS working in manual testing

• Subject Token needs to be query escaped
• Null options need to be omitted (like they are in other languages)

Change-Id: I67d1ed3ba96a35283a8928f196bc7e912084d1ab
GitHub-Last-Rev: 1aae076a90
GitHub-Pull-Request: golang/oauth2#474
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/290513
Reviewed-by: Cody Oss <codyoss@google.com>
Trust: Cody Oss <codyoss@google.com>
Trust: Tyler Bui-Palsulich <tbp@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

google/internal/externalaccount/aws_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	neturl "net/url"
 	"reflect"
 	"strings"
 	"testing"
@@ -527,7 +528,7 @@ func getExpectedSubjectToken(url, region, accessKeyID, secretAccessKey, security
 	})
 
 	str, _ := json.Marshal(result)
-	return string(str)
+	return neturl.QueryEscape(string(str))
 }
 
 func TestAwsCredential_BasicRequest(t *testing.T) {