Fixing a type error.
Fixing a type error.

This is done to fix a build error when writing tests in TypeScript.
The error is due to wrong type inference, so we need
to explicitly cast the response from the send() method.

Change-Id: Ied4627e73a6ecf7b6f06da229abb8653e16e2d3d

actions-on-google/actions-on-google-testing-nodejs,actions-on-google/actions-on-google-testing-nodejs,actions-on-google/actions-on-google-testing-nodejs