Need test for educatee's handle in tweet content

[McEileen]

As mentioned in the discussion in PR #147, we do not have any tests that confirm the response tweet content includes an @ before the educatee's handle. I'm assigning myself to this issue because I originally worked on the code that requires this test, but I'm also open to contributions from others on it. See this article about Minitest for a helpful discussion of mocking and stubbing.