Warn when using user credentials from the Cloud SDK (#266)

diff --git a/tests/test__default.py b/tests/test__default.py
index 68c4fb0..d7d537c 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -33,6 +33,9 @@
 with open(AUTHORIZED_USER_FILE) as fh:
     AUTHORIZED_USER_FILE_DATA = json.load(fh)
 
+AUTHORIZED_USER_CLOUD_SDK_FILE = os.path.join(
+    DATA_DIR, 'authorized_user_cloud_sdk.json')
+
 SERVICE_ACCOUNT_FILE = os.path.join(DATA_DIR, 'service_account.json')
 
 with open(SERVICE_ACCOUNT_FILE) as fh:
@@ -88,6 +91,14 @@
     assert excinfo.match(r'missing fields')
 
 
+def test__load_credentials_from_file_authorized_user_cloud_sdk():
+    with pytest.warns(UserWarning, matches='Cloud SDK'):
+        credentials, project_id = _default._load_credentials_from_file(
+            AUTHORIZED_USER_CLOUD_SDK_FILE)
+    assert isinstance(credentials, google.oauth2.credentials.Credentials)
+    assert project_id is None
+
+
 def test__load_credentials_from_file_service_account():
     credentials, project_id = _default._load_credentials_from_file(
         SERVICE_ACCOUNT_FILE)