iOS: Support HTTP headers for source prop on <Image> components

Summary:
Allows developers to specify headers to include in the HTTP request
when fetching a remote image. For example, one might leverage this
when fetching an image from an endpoint that requires authentication:

```
<Image
  style={styles.logo}
  source={{
    uri: 'http://facebook.github.io/react/img/logo_og.png',
    headers: {
      Authorization: 'someAuthToken'
    }
  }}
/>
```

Note that the header values must be strings.

Works on iOS and Android.

**Test plan (required)**

- Ran a small example like the one above on iOS and Android and ensured the headers were sent to the server.
- Ran a small example to ensure that \<Image\> components without headers still work.
- Currently using this code in our app.

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/7338

Reviewed By: javache

Differential Revision: D3371458

Pulled By: nicklockwood

fbshipit-source-id: cdb24fe2572c3ae3ba82c86ad383af6d85157e20

RCTCameraRollManager.m

@@ -82,11 +82,12 @@ RCT_EXPORT_MODULE()
 NSString *const RCTErrorUnableToLoad = @"E_UNABLE_TO_LOAD";
 NSString *const RCTErrorUnableToSave = @"E_UNABLE_TO_SAVE";
 
-RCT_EXPORT_METHOD(saveImageWithTag:(NSString *)imageTag
+RCT_EXPORT_METHOD(saveImageWithTag:(NSURLRequest *)imageRequest
                   resolve:(RCTPromiseResolveBlock)resolve
                   reject:(RCTPromiseRejectBlock)reject)
 {
-  [_bridge.imageLoader loadImageWithTag:imageTag callback:^(NSError *loadError, UIImage *loadedImage) {
+  [_bridge.imageLoader loadImageWithURLRequest:imageRequest
+                                      callback:^(NSError *loadError, UIImage *loadedImage) {
     if (loadError) {
       reject(RCTErrorUnableToLoad, nil, loadError);
       return;