google: Use bytes.Buffer for go 1.9 compatability

Update test cases to use const test data file references.

Change-Id: Ic08b6de5a84db7b2ae2c649ee676c5a2c13f4227
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/306749
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.go

@@ -5,6 +5,7 @@
 package externalaccount
 
 import (
+	"bytes"
 	"context"
 	"crypto/hmac"
 	"crypto/sha256"
@@ -127,7 +128,7 @@ func canonicalHeaders(req *http.Request) (string, string) {
 	}
 	sort.Strings(headers)
 
-	var fullHeaders strings.Builder
+	var fullHeaders bytes.Buffer
 	for _, header := range headers {
 		headerValue := strings.Join(lowerCaseHeaders[header], ",")
 		fullHeaders.WriteString(header)