WASHINGTON (Reuters) - The U.S. economy slowed as expected in the first quarter, but a less robust pace of consumer spending and export growth than previously estimated could dampen the economic outlook for the current period. Gross domestic product increased at a 1.9 percent annual rate, the Commerce Department said in its final reading on Thursday, unchanged from its estimate last month. That was in line with economists' expectations. However, when measured from the income side, the economy grew at a 3.1 percent pace in the first quarter, up from 2.6 percent in the previous quarter. ...