refactor: the default value of tipFormatter

src/Handle.jsx

@@ -42,7 +42,7 @@ export default class Handle extends React.Component {
               prefixCls={className.replace('slider-handle', 'tooltip')}
               placement="top"
               visible={isTooltipVisible}
-              overlay={<span>{tipFormatter ? tipFormatter(value) : value}</span>}
+              overlay={<span>{tipFormatter(value)}</span>}
               delay={0}
               transitionName={tipTransitionName}>
               {handle}

src/Slider.jsx

@@ -417,6 +417,7 @@ Slider.defaultProps = {
   onBeforeChange: noop,
   onChange: noop,
   onAfterChange: noop,
+  tipFormatter: value => value,
   included: true,
   disabled: false,
   dots: false,