Handle the case where redirect URL is relative

Summary:
While trying to download an image I encountered an error:

![screen shot 2016-05-31 at 13 58 54](https://cloud.githubusercontent.com/assets/10407025/15799726/a7bcbf1c-2a5d-11e6-926a-b2f84e011258.png)

As you can see the domain name is missing from the URL. I traced it back to a method which handles redirects. It would retrieve the redirect URL from the `Location` header field, but it didn’t handle the case where `Location` contains a relative URL (which [according to the RFC spec](https://tools.ietf.org/html/rfc7231#section-7.1.2), is allowed).
Closes https://github.com/facebook/react-native/pull/7926

Differential Revision: D3399531

Pulled By: nicklockwood

fbshipit-source-id: ffbd5e9fc55b1737a8ff6a9bcc06fb1f9f19d093

Libraries/Image/RCTImageLoader.m

@@ -397,7 +397,7 @@ static UIImage *RCTResizeImageIfNeeded(UIImage *image,
             return;
           }
 
-          NSURL *redirectURL = [NSURL URLWithString: location];
+          NSURL *redirectURL = [NSURL URLWithString: location relativeToURL: request.URL];
           request = [NSURLRequest requestWithURL:redirectURL];
           cachedResponse = [_URLCache cachedResponseForRequest:request];
           continue;