Shoppers stepped up their spending in May, surpassing economists’ expectations, as temperatures warmed and gasoline prices leveled off. Retail sales rose 0.9%, up from a revised 0.4% gain in April, according to new Commerce Department data released Wednesday. Sales got a boost from generous government tax refunds in both April and May, though economists say that cushion is starting to fade. Excluding sales at gas stations, retail sales in May rose 0.7%. Spending was broad-based. Business at miscellaneous stores, clothing and accessories stores and home furnishing and furniture stores all posted increases. Online sales rose 1.5%.