util.async: Use wrapper for once runner (thanks luacheck)

spec/util_async_spec.lua

@@ -601,6 +601,7 @@ describe("util.async", function()
 			end);
 			async.once(f);
 			assert.spy(f).was.called();
+			assert.spy(f).was.called_with();
 		end);
 		it("should propagate errors", function ()
 			local function should_error()

util/async.lua

@@ -231,7 +231,7 @@ do
 	};
 	local function once_runner(func) func(); end
 	function once(func)
-		local r = runner(func, once_watchers);
+		local r = runner(once_runner, once_watchers);
 		return r:run(func);
 	end
 end