Support for repeated key value pairs in URL encoded forms

By using NSSet for a key in the dictionary we can workaround the fact that an NSDictionary requires unique keys and allow for repeated key value pairs by using an NSSet of the values. This maintains support for true arrays and allows supporting of APIs that make use of repeated values.

Related to: http://stackoverflow.com/questions/9713923/afnetworking-overload-a-post-parameter