Call queryStringSerialization block of the AFHTTPRequestSerializer even if request parameters are nil

Sometimes it may be useful to pass some parameters in the query string by default (timestamp for example) while creating the request. So `AFHTTPRequestSerializer` should call `queryStringSerialization` block (if it's set of course) even if there are no query parameters.
Moreover now the parameters are checked only for a `nil` and for empty containers (`NSDictionary` etc) block is called (it's a kind of workaround but in my opinion it's not right).