convert header value to str in case its an int or other simple type.

azure-devops/azure/devops/client.py

@@ -91,7 +91,7 @@ def _send(self, http_method, location_id, version, route_values=None,
                    'Accept': accept_media_type + ';api-version=' + negotiated_version}
         if additional_headers is not None:
             for key in additional_headers:
-                headers[key] = additional_headers[key]
+                headers[key] = str(additional_headers[key])
         if self.config.additional_headers is not None:
             for key in self.config.additional_headers:
                 headers[key] = self.config.additional_headers[key]