把 style 里面的 background 属性转换成简写形式, 用于减少代码
function() {
        var background = '',
            style = this;
        if(style.getPropertyValue('background')) {
			return;
		}
        var positionText = this.removeProperty('background-position-x') + ' ' +
            this.removeProperty('background-position-y');

        style.setProperty('background-position', positionText.trim(), null);

        var toMergeAttrs = [
            'background-color', 'background-image', 'background-position',
            'background-repeat', 'background-attachment',
            'background-origin', 'background-clip'
        ];
        for (var i = 0, item; item = toMergeAttrs[i]; i++) {
            if (style.hasOwnProperty(item)) {
                background += this.removeProperty(item) + ' ';
            }
        }
        if(background.trim()) {
            style.setProperty('background', background.trim(), null);
        }
    }