Fix textarea not editable bug, close #646

Caused by #589 fix

and fix #586 via https://github.com/facebook/react/issues/2533#issuecomment-15150334

components/input/index.jsx

 import React from 'react';
+import assign from 'object-assign';
 
 function prefixClsFn(prefixCls, ...args) {
   return args.map((s)=> {
@@ -14,6 +15,13 @@ function ieGT9() {
   return documentMode > 9;
 }
 
+function fixControlledValue(value) {
+  if (typeof value === 'undefined' || value === null) {
+    return '';
+  }
+  return value;
+}
+
 class Group extends React.Component {
   render() {
     const className = 'ant-input-group ' + (this.props.className || '');
@@ -57,7 +65,7 @@ class Input extends React.Component {
   }
 
   renderInput() {
-    const props = this.props;
+    const props = assign({}, this.props);
     const prefixCls = props.prefixCls;
     let inputClassName = prefixClsFn(prefixCls, 'input');
     if (!props.type) {
@@ -73,9 +81,12 @@ class Input extends React.Component {
     if(placeholder && ieGT9()){
       placeholder = null;
     }
+    if ('value' in props) {
+      props.value = fixControlledValue(props.value);
+    }
     switch (props.type) {
     case 'textarea':
-      return <textarea {...props} value={props.value || props.defaultValue} placeholder={placeholder} className={inputClassName} ref="input" />;
+      return <textarea {...props} placeholder={placeholder} className={inputClassName} ref="input" />;
     default:
       inputClassName = props.className ? props.className : inputClassName;
       return <input {...props} placeholder={placeholder} className={inputClassName} ref="input"/>;