Fix build under 10.7 SDK

This is the sole use of the [] operator on NSDictionary, introduced in 1.2.0. It doesn't build when Base SDK is set to 10.7.

AFNetworking/AFHTTPClient.m

@@ -150,7 +150,7 @@ NSArray * AFQueryStringPairsFromKeyAndValue(NSString *key, id value) {
         // Sort dictionary keys to ensure consistent ordering in query string, which is important when deserializing potentially ambiguous sequences, such as an array of dictionaries
         NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"description" ascending:YES selector:@selector(caseInsensitiveCompare:)];
         for (id nestedKey in [dictionary.allKeys sortedArrayUsingDescriptors:@[ sortDescriptor ]]) {
-            id nestedValue = dictionary[nestedKey];
+            id nestedValue = [dictionary objectForKey:nestedKey];
             if (nestedValue) {
                 [mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue((key ? [NSString stringWithFormat:@"%@[%@]", key, nestedKey] : nestedKey), nestedValue)];
             }