Dryer flatten arguments

The following two commits accomplish the following, respectively:

7c9d39b:
Certain utility methods in payload, when reworked as class methods, can be used in other parts of rest-client to ensure that parameters are consistently flattened into HTTP requests. By itself, this commit is functionally equivalent to its ancestor.

593cf00:
By utilizing Payload::UrlEncoded.flatten_params instead of the current ad-hoc implementation in Request#process_url_params, we're able to resolve the following issue with existing code. This allows requests to be created with arrays, using the Rails convention already adopted by Payload.rb.

https://github.com/rest-client/rest-client/issues/59