By Ben Klayman DETROIT, July 31 (Reuters) - Delphi Automotive Plc on Wednesday posted a stronger-than-expected second-quarter profit, but the auto parts supplier offered a weaker-than-anticipated forecast for the current quarter. Analysts said investors were disappointed that the outperformance in the second quarter was not fully incorporated in the company's improved full-year forecast. "You had a very good quarter, very strong margins, in-line revenue, but the upside in the quarte