fix get_case_observables method

[billmurrin]

With the most recent version of TheHive4Py, trying to retrieve observables programmatically via the get_case_observables method was throwing the following error:

{u'message': u'JsResultException(errors:List((,List(JsonValidationError(List(Invalid query: unexpected [{"_parent":{"_type":"case","_query":{"_id":"AWA9SlYM_XlqkMKDzIB0"}}},{"_value":"Ok","_field":"status"}]),WrappedArray())))))', u'type': u'play.api.libs.json.JsResultException'}

I updated the code using the same structure present in the get_case_tasks method.

[nadouani]

Thanks, in fact the And methods uses args not an array as params.