Allow for string `params`

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