Implemented response headers when using XMLHttpRequest

Summary:
I think perhaps these were left out by mistake?
Closes https://github.com/facebook/react-native/pull/382
Github Author: Mike Driver <mikedriver@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Base/RCTConvert.h

@@ -38,6 +38,7 @@
 + (NSDictionary *)NSDictionary:(id)json;
 + (NSString *)NSString:(id)json;
 + (NSNumber *)NSNumber:(id)json;
++ (NSData *)NSData:(id)json;
 
 + (NSURL *)NSURL:(id)json;
 + (NSURLRequest *)NSURLRequest:(id)json;

React/Base/RCTConvert.m

@@ -50,6 +50,12 @@ RCT_CONVERTER(NSString *, NSString, description)
   return nil;
 }
 
++ (NSData *)NSData:(id)json
+{
+  // TODO: should we automatically decode base64 data? Probably not...
+  return [[self NSString:json] dataUsingEncoding:NSUTF8StringEncoding];
+}
+
 + (NSURL *)NSURL:(id)json
 {
   if (![json isKindOfClass:[NSString class]]) {