Don't use integrate_polynomial_kt in integrate_hypertangent

The input to integrate_polynomial should be, as the name suggests, a
polynomial.

This adds a test, and also fixes a wrong test (the integral of
(x*tan(x/2)**2 + 1)/tan(x/2) is not elementary, see
http://www.wolframalpha.com/input/?i=integrate%28%28x*tan%28x%2F2%29**2+%2B+1%29%2Ftan%28x%2F2%29%29).