fix: only add IAM scope to credentials that can change scopes (#451)

diff --git a/system_tests/conftest.py b/system_tests/conftest.py
index 1893007..02de846 100644
--- a/system_tests/conftest.py
+++ b/system_tests/conftest.py
@@ -25,6 +25,9 @@
 
 HERE = os.path.dirname(__file__)
 DATA_DIR = os.path.join(HERE, "data")
+IMPERSONATED_SERVICE_ACCOUNT_FILE = os.path.join(
+    DATA_DIR, "impersonated_service_account.json"
+)
 SERVICE_ACCOUNT_FILE = os.path.join(DATA_DIR, "service_account.json")
 AUTHORIZED_USER_FILE = os.path.join(DATA_DIR, "authorized_user.json")
 URLLIB3_HTTP = urllib3.PoolManager(retries=False)
@@ -40,6 +43,12 @@
 
 
 @pytest.fixture
+def impersonated_service_account_file():
+    """The full path to a valid service account key file."""
+    yield IMPERSONATED_SERVICE_ACCOUNT_FILE
+
+
+@pytest.fixture
 def authorized_user_file():
     """The full path to a valid authorized user file."""
     yield AUTHORIZED_USER_FILE
diff --git a/system_tests/noxfile.py b/system_tests/noxfile.py
index e37049e..8110632 100644
--- a/system_tests/noxfile.py
+++ b/system_tests/noxfile.py
@@ -170,7 +170,8 @@
 # Test sesssions
 
 TEST_DEPENDENCIES = ["pytest", "requests"]
-PYTHON_VERSIONS=['2.7', '3.7']
+PYTHON_VERSIONS = ["2.7", "3.7"]
+
 
 @nox.session(python=PYTHON_VERSIONS)
 def service_account(session):
@@ -187,6 +188,13 @@
 
 
 @nox.session(python=PYTHON_VERSIONS)
+def impersonated_credentials(session):
+    session.install(*TEST_DEPENDENCIES)
+    session.install(LIBRARY_DIR)
+    session.run("pytest", "test_impersonated_credentials.py")
+
+
+@nox.session(python=PYTHON_VERSIONS)
 def default_explicit_service_account(session):
     session.env[EXPLICIT_CREDENTIALS_ENV] = SERVICE_ACCOUNT_FILE
     session.env[EXPECT_PROJECT_ENV] = "1"
diff --git a/system_tests/secrets.tar.enc b/system_tests/secrets.tar.enc
index 1106f8a..af10c71 100644
--- a/system_tests/secrets.tar.enc
+++ b/system_tests/secrets.tar.enc
Binary files differ
diff --git a/system_tests/test_impersonated_credentials.py b/system_tests/test_impersonated_credentials.py
new file mode 100644
index 0000000..6689e89
--- /dev/null
+++ b/system_tests/test_impersonated_credentials.py
@@ -0,0 +1,99 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+import pytest
+
+import google.oauth2.credentials
+from google.oauth2 import service_account
+import google.auth.impersonated_credentials
+from google.auth import _helpers
+
+
+GOOGLE_OAUTH2_TOKEN_ENDPOINT = "https://oauth2.googleapis.com/token"
+
+
+@pytest.fixture
+def service_account_credentials(service_account_file):
+    yield service_account.Credentials.from_service_account_file(service_account_file)
+
+
+@pytest.fixture
+def impersonated_service_account_credentials(impersonated_service_account_file):
+    yield service_account.Credentials.from_service_account_file(
+        impersonated_service_account_file
+    )
+
+
+def test_refresh_with_user_credentials_as_source(
+    authorized_user_file,
+    impersonated_service_account_credentials,
+    http_request,
+    token_info,
+):
+    with open(authorized_user_file, "r") as fh:
+        info = json.load(fh)
+
+    source_credentials = google.oauth2.credentials.Credentials(
+        None,
+        refresh_token=info["refresh_token"],
+        token_uri=GOOGLE_OAUTH2_TOKEN_ENDPOINT,
+        client_id=info["client_id"],
+        client_secret=info["client_secret"],
+        # The source credential needs this scope for the generateAccessToken request
+        # The user must also have `Service Account Token Creator` on the project
+        # that owns the impersonated service account.
+        # See https://cloud.google.com/iam/docs/creating-short-lived-service-account-credentials
+        scopes=["https://www.googleapis.com/auth/cloud-platform"],
+    )
+
+    source_credentials.refresh(http_request)
+
+    target_scopes = [
+        "https://www.googleapis.com/auth/devstorage.read_only",
+        "https://www.googleapis.com/auth/analytics",
+    ]
+    target_credentials = google.auth.impersonated_credentials.Credentials(
+        source_credentials=source_credentials,
+        target_principal=impersonated_service_account_credentials.service_account_email,
+        target_scopes=target_scopes,
+        lifetime=100,
+    )
+
+    target_credentials.refresh(http_request)
+    assert target_credentials.token
+
+
+def test_refresh_with_service_account_credentials_as_source(
+    http_request,
+    service_account_credentials,
+    impersonated_service_account_credentials,
+    token_info,
+):
+    source_credentials = service_account_credentials.with_scopes(["email"])
+    source_credentials.refresh(http_request)
+    assert source_credentials.token
+
+    target_scopes = [
+        "https://www.googleapis.com/auth/devstorage.read_only",
+        "https://www.googleapis.com/auth/analytics",
+    ]
+    target_credentials = google.auth.impersonated_credentials.Credentials(
+        source_credentials=source_credentials,
+        target_principal=impersonated_service_account_credentials.service_account_email,
+        target_scopes=target_scopes,
+    )
+
+    target_credentials.refresh(http_request)
+    assert target_credentials.token