Make AFQueryStringFromParametersWithEncoding() available in the .h file

I'm have written an encrypted request serializer that subclasses `AFHTTPRequestSerializer`. For this I call the `AFQueryStringFromParametersWithEncoding()` function.

This `AFQueryStringFromParametersWithEncoding()` function is currently fully encapsulated inside the `AFURLRequestSerialization.m` file. In order to gain access to it I had to remove `static` and expose it in the .h file.

Thank you for an awesome network framework. Have a great day.