Update XHRExampleFetch and dump the response headers

Summary:
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Dump the response headers so that we can check `Set-Cookie` header is working properly whether or not.

**Test plan (required)**

Manual Test.

![image](https://cloud.githubusercontent.com/assets/104052/18259326/6b86c90a-7415-11e6-8979-92bd24ac703c.png)

**Code formatting**

Look around. Match the style of the rest of the codebase. See also the simple [style guide](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#style-guide).

For more info, see the ["Pull Requests" section of our "Contributing" guidelines](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests).
Closes https://github.com/facebook/react-native/pull/9687

Differential Revision: D3821145

Pulled By: javache

fbshipit-source-id: 24baa051165d3b451ba0e71efbe70db44b0bf45e

Examples/UIExplorer/js/XHRExampleFetch.js

@@ -36,6 +36,7 @@ var {
 class XHRExampleFetch extends React.Component {
   state: any;
   responseURL: ?string;
+  responseHeaders: ?Object;
 
   constructor(props: any) {
     super(props);
@@ -43,17 +44,34 @@ class XHRExampleFetch extends React.Component {
      responseText: null,
     };
     this.responseURL = null;
+    this.responseHeaders = null;
   }
 
   submit(uri: String) {
     fetch(uri).then((response) => {
       this.responseURL = response.url;
+      this.responseHeaders = response.headers;
       return response.text();
     }).then((body) => {
       this.setState({responseText: body});
     });
   }
 
+  _renderHeaders() {
+    if (!this.responseHeaders) {
+      return null;
+    }
+
+    var responseHeaders = [];
+    var keys = Object.keys(this.responseHeaders.map);
+    for (var i = 0; i < keys.length; i++) {
+      var key = keys[i];
+      var value = this.responseHeaders.get(key);
+      responseHeaders.push(<Text>{key}: {value}</Text>);
+    }
+    return responseHeaders;
+  }
+
   render() {
 
     var responseURL = this.responseURL ? (
@@ -63,6 +81,13 @@ class XHRExampleFetch extends React.Component {
       </View>
     ) : null;
 
+    var responseHeaders = this.responseHeaders ? (
+      <View style={{marginTop: 10}}>
+        <Text style={styles.label}>Server response headers:</Text>
+        {this._renderHeaders()}
+      </View>
+    ) : null;
+
     var response = this.state.responseText ? (
       <View style={{marginTop: 10}}>
         <Text style={styles.label}>Server response:</Text>
@@ -87,6 +112,7 @@ class XHRExampleFetch extends React.Component {
           style={styles.textInput}
         />
         {responseURL}
+        {responseHeaders}
         {response}
       </View>
     );