Update responsetypes.py

The problem creator should be able to specify a different correct answer to be reported to the student via correct_answer, so that expect can be something weird.  For example, what if the answer is a regular expression that the check function uses to determine whether the student's answer is one of several correct possibilities?  The regular expression itself is likely not going to be very informative to the student as a correct answer.  I _believe_ the change I have proposed does this without breaking old problems, as it defaults to using expect if correct_answer is not specified.

I've never done anything with github before, so I hope I'm not doing something bizarre by proposing this change in this way...