Fix cascader placeholder, close #1312

components/cascader/demo/basic.md

@@ -37,6 +37,6 @@ function onChange(value) {
 }
 
 ReactDOM.render(
-  <Cascader options={options} onChange={onChange} />
+  <Cascader options={options} onChange={onChange} placeholder="请选择地区" />
 , mountNode);
 ````

components/cascader/index.jsx

@@ -8,7 +8,7 @@ import classNames from 'classnames';
 export default class Cascader extends React.Component {
   static defaultProps = {
     prefixCls: 'ant-cascader',
-    placeholder: '请选择',
+    placeholder: 'Please select',
     transitionName: 'slide-up',
     popupPlacement: 'bottomLeft',
     onChange() {},