(Adds head of Americas Research at Lipper, table) By Sam Forgione NEW YORK, Aug 20 (Reuters) - Investors in U.S.-based funds poured $1.3 billion into funds that specialize in U.S. Treasuries in the week ended Aug. 19, marking the funds' biggest weekly inflows since mid-April, data from Thomson Reuters' Lipper service showed on Thursday. Taxable bond funds overall attracted their first inflows in four weeks at $606 million. funds that specialize in investment-grade corporate bonds