Added logic to detect text encoding for downloaded data

Summary: @​public

RCTNetworking currently relies on network responses to include an accurate text encoding, otherwise it is unable to convert the response data to text unless it's encoded as UTF8.

See: https://github.com/facebook/react-native/issues/1780#issuecomment-139334294 for details.

This diff makes use of a new feature in iOS8 to detect the encoding of the text authomatically

Reviewed By: @sahrens

Differential Revision: D2443446

Examples/UIExplorer/XHRExample.ios.js

@@ -302,6 +302,53 @@ class FormUploader extends React.Component {
   }
 }
 
+class FetchTest extends React.Component {
+
+  constructor(props) {
+    super(props);
+    this.state = {
+     responseText: null,
+    };
+  }
+
+  submit(uri: String) {
+    fetch(uri).then((response) => {
+      return response.text();
+    }).then((body) => {
+      this.setState({responseText: body});
+    });
+  }
+
+  render() {
+
+    var response = this.state.responseText ? (
+      <View style={{marginTop: 10}}>
+        <Text style={styles.label}>Server response:</Text>
+        <TextInput
+          editable={false}
+          multiline={true}
+          defaultValue={this.state.responseText}
+          style={styles.textOutput}
+        />
+      </View>
+    ) : null;
+
+    return (
+      <View>
+        <Text style={styles.label}>Edit URL to submit:</Text>
+        <TextInput
+          returnKeyType="go"
+          defaultValue="http://www.posttestserver.com/post.php"
+          onSubmitEditing={(event)=> {
+            this.submit(event.nativeEvent.text);
+          }}
+          style={styles.textInput}
+        />
+        {response}
+      </View>
+    );
+  }
+}
 
 exports.framework = 'React';
 exports.title = 'XMLHttpRequest';
@@ -316,6 +363,11 @@ exports.examples = [{
   render() {
     return <FormUploader/>;
   }
+}, {
+  title: 'fetch test',
+  render() {
+    return <FetchTest/>;
+  }
 }];
 
 var styles = StyleSheet.create({
@@ -373,4 +425,19 @@ var styles = StyleSheet.create({
     fontSize: 16,
     fontWeight: '500',
   },
+  label: {
+    flex: 1,
+    color: '#aaa',
+    fontWeight: '500',
+    height: 20,
+  },
+  textOutput: {
+    flex: 1,
+    fontSize: 17,
+    borderRadius: 3,
+    borderColor: 'grey',
+    borderWidth: 1,
+    height: 200,
+    paddingLeft: 8,
+  },
 });