Since the definition of _request is function _request(verb, path, params::Dict, files::Dict, data::Dict, json::Dict) it means, that it accepts only positional arguments, so _request call in _get should look like
return _request(GET, path, kwargs)
If you want to use keyword arguments you should write them after ; and provide some default value. For example
function _request(verb, path, files::Dict, data::Dict, json::Dict; params = Dict())
More information can be found in functions documentation.