Corrects the Differential Extension:

     The following PR corrects the differential extension of the test
case in risch.py under test_recognize_log_derivative. It changes
DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(1, t)]}) to
DE = DifferentialExtension(extension={'D': [Poly(1, x)]}) and also adds
the case where DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(t**2 + 1, t]})
Merge pull request #2384 from cheatiiit/correct_predict

DE in test_risch.py (test_recognize_log_derivative)