Fix TextInput 'defaultValue' propTypes to 'string'

Summary:
Because `TextInput` only render the `defaultValue` of `string`, when using other types of values, it will render empty('') without any Warnings.
Closes https://github.com/facebook/react-native/pull/11478

Differential Revision: D4340132

Pulled By: lacker

fbshipit-source-id: aedb96d49277836000c0adc53007c97db6363253

Libraries/Components/TextInput/TextInput.js

@@ -452,7 +452,7 @@ const TextInput = React.createClass({
      * Useful for simple use-cases where you do not want to deal with listening
      * to events and updating the value prop to keep the controlled state in sync.
      */
-    defaultValue: PropTypes.node,
+    defaultValue: PropTypes.string,
     /**
      * When the clear button should appear on the right side of the text view.
      * @platform ios