AFHTTPClient Registered operation classes consulted in wrong order.

The documentation states registered operation classes are consulted to the reverse order in which they are registered. New registrations are added to the beginning of the array in the `registerHTTPOperationClass:` implementation. 

Fixed using the reverseObjectEnumerator in `HTTPRequestOperationWithRequest:success:failure:` when iterating the `registeredHTTPOperationClassNames` array which produced the opposite of documented behaviour. fixes #794

The ordering of the array should be associated with the register implementation. The array should not have to be reverse iterated for each method that might require it. That is why removed the use of the `reverseObjectEnumerator`