Currently RestWorkItemHandler is ignoring the query part and realy on parameters of the form QUERY_<query_param_name> to fill the query part of the URI.
In order to make it easier copy pasting a URI for Custom REST operation in serverless workflow, for example, to be able to write this

rest:get:https://api.agify.io:80/?name={name}

and specify this arguments object

{
 "name" : "Pepe"
}

which will be an alternative to the currently working, but somehow more cumbersome

rest:get:https://api.agify.io:80/

with params

{
"QUERY_name": "Pepe"
}