fix: set Content-Type header in the request to signBlob API to avoid Invalid JSON payload error

google/auth/iam.py

@@ -69,7 +69,7 @@ def _make_signing_request(self, message):
 
         method = "POST"
         url = _SIGN_BLOB_URI.format(self._service_account_email)
-        headers = {}
+        headers = {"Content-Type": "application/json"}
         body = json.dumps(
             {"bytesToSign": base64.b64encode(message).decode("utf-8")}
         ).encode("utf-8")

tests/test_iam.py

@@ -86,6 +86,8 @@ def test_sign_bytes(self):
         returned_signature = signer.sign("123")
 
         assert returned_signature == signature
+        kwargs = request.call_args.kwargs
+        assert kwargs.get("headers").get("Content-Type") == "application/json"
 
     def test_sign_bytes_failure(self):
         request = make_request(http_client.UNAUTHORIZED)