Handled flatten_params_array for empty arrays

If flatten_params encounters an empty array, it skipped it. Now it behaves similar to an empty string (which works as expected). Added tests in spec illustrating the change. 

Scenario: I was working on an app where I need to return a list of selected items (among other things). If nothing is selected, rest-client skips the array giving the server the impression nothing has changed.

This patch worked for me, is there a better to handle this?