test: add more tests for service account quota projects (#527)

Follow up to #519 
diff --git a/tests/oauth2/test_credentials.py b/tests/oauth2/test_credentials.py
index 69d9fbc..ceb8cdf 100644
--- a/tests/oauth2/test_credentials.py
+++ b/tests/oauth2/test_credentials.py
@@ -309,6 +309,7 @@
         headers = {}
         creds.apply(headers)
         assert headers["x-goog-user-project"] == "quota-project-123"
+        assert "token" in headers["authorization"]
 
     def test_apply_with_no_quota_project_id(self):
         creds = credentials.Credentials(
@@ -322,6 +323,7 @@
         headers = {}
         creds.apply(headers)
         assert "x-goog-user-project" not in headers
+        assert "token" in headers["authorization"]
 
     def test_with_quota_project(self):
         creds = credentials.Credentials(