WASHINGTON (Reuters) - U.S. consumer spending rebounded by the most on record in May, but the gains are not likely to be sustainable, with income dropping and expected to decline further as millions lose their unemployment checks starting next month.The Commerce Department said on Friday consumer spending, which accounts for more than two-thirds of U.S. economic activity, jumped 8.2% last month. That was largest increase since the government started tracking the series in 1959. Consumer spending..