An Open, Single-Centre, Randomized, 6-way Crossover, Dose-Response Comparative Study of Esomeprazole 20, 40 and 80 mg and Pantoprazole 20, 40 and 80 mg regarding 24-hour Intragastric pH following 5 Days Repeated Oral Dose Administration in Patients with Symptoms of Gastroesophageal Reflux Disease