Fix nested user object in iOS with userCallback response

Author: chrisbianca

ios/Firestack/FirestackAuth.m

@@ -480,9 +480,7 @@ - (void) sendJSEvent:(NSString *)title
 
 - (void) userCallback:(RCTResponseSenderBlock) callback
                  user:(FIRUser *) user {
-    NSDictionary *userProps = @{
-                                @"user": [self userPropsFromFIRUser:user]
-                                };
+    NSDictionary *userProps = [self userPropsFromFIRUser:user];
     callback(@[[NSNull null], userProps]);
 }