Resolves issue w/ http_build_query on certain server configurations

See  the following URL for a forum post regarding this bug. 

http://community.constantcontact.com/t5/Getting-Started-with-API-s/I-found-a-bug-in-the-PHP-SDK-and-have-a-bug-fix/td-p/77745

This pull request will explicitly specify '&' as the url separator vs leaving it up to server configuration.