WASHINGTON — With less than hour to spare, the Senate late Friday backed legislation averting a government shutdown as coal-state Democrats retreated on long-term health care benefits for retired miners but promised a renewed fight for the working class next year. The vote was 63-36 and sent the stop-gap spending bill to President Barack Obama for his signature ahead of a midnight deadline. It came hours after Democrats dropped threats to block the measure in hopes of using the shutdown deadline to try to win a one-year respite for 16,500 miners facing the loss of health care benefits at year’s end.