runHandlers = function (queue, context, args, isHead) {
			var len = queue.length,
				i = 0,
				tmp;

			for (; i < len; ++i) {
				tmp = queue[i].apply(context, args);
				if (tmp !== UNDEFINED) {
					args = isHead ? tmp : [ tmp ];
				}
			}

			return isHead ? args : args[0];
		}
Run head&tail handlers serially.
@param queue {Array}
@param context {Object}
@param isHead {boolean}
@param ret {*}