Use isEqual: to determine if value is NSNull

Author: mattt

AFNetworking/AFHTTPClient.m

@@ -473,7 +473,7 @@ - (NSMutableURLRequest *)multipartFormRequestWithMethod:(NSString *)method
             NSData *data = nil;
             if ([pair.value isKindOfClass:[NSData class]]) {
                 data = pair.value;
-            } else if ([pair.value isKindOfClass:[NSNull class]]) {
+            } else if ([pair.value isEqual:[NSNull null]]) {
                 data = [NSData data];
             } else {
                 data = [[pair.value description] dataUsingEncoding:self.stringEncoding];