Allow for string params

[nicoloverardo]

The Resource, under the method add_action, will try to .update() the request.params, implying that params should be a dict. There are cases in which one would want to manually pass a str as params instead of dict, and therefore this would fail.

https://github.com/allisson/python-simple-rest-client/blob/master/simple_rest_client/resource.py#L102