google: manual testing fixes

I found some errors while manually testing service account impersonation on Azure.  This PR includes the fixes that I made.

Change-Id: Ia2b194be6c9a7c843e615f9789c8f8203bcbc151
GitHub-Last-Rev: 56907163634bb65457c12320b4081e919cfbbcb2
GitHub-Pull-Request: golang/oauth2#475
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/291209
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/urlcredsource_test.go

@@ -7,6 +7,7 @@ package externalaccount
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -19,11 +20,18 @@ func TestRetrieveURLSubjectToken_Text(t *testing.T) {
 		if r.Method != "GET" {
 			t.Errorf("Unexpected request method, %v is found", r.Method)
 		}
+		fmt.Println(r.Header)
+		if r.Header.Get("Metadata") != "True" {
+			t.Errorf("Metadata header not properly included.")
+		}
 		w.Write([]byte("testTokenValue"))
 	}))
+	heads := make(map[string]string)
+	heads["Metadata"] = "True"
 	cs := CredentialSource{
 		URL:    ts.URL,
 		Format: format{Type: fileTypeText},
+		Headers: heads,
 	}
 	tfc := testFileConfig
 	tfc.CredentialSource = cs