A multi-centre, randomized, double-blind, active control, parallel-group, 2-arm study to investigate the effect of ethinylestradiol/drospirenone/(0.02 mg/3 mg) oral contraception in a 24/4 regimen compared to ethinylestradiol/desogestrel (0.02 mg/0.15 mg) oral contraception in a 21/7 regimen on hormone withdrawal associated symptoms in otherwise healthy women after 4 cycles of treatment