Steps to Reproduce
SendGrid sg = new SendGrid(sendGridApiKey);
Request request = new Request();
request.setMethod(Method.GET);
request.setEndpoint("categories/stats");
request.addQueryParam("aggregated_by", "day");
request.addQueryParam("start_date", "2021-09-06");;
request.addQueryParam("categories", "category1&category2");
Response response = sg.api(request);
The above code makes a GET to this URL. Sendgrid returns a 404 for this request.

When utilizing the code it reproduces this url:

https://api.sendgrid.com/v3/categories/stats?start_date=2021-09-01&categories=Promotions%26Survey&limit=500&aggregated_by=day

When it should be producing this url:

https://api.sendgrid.com/v3/categories/stats?start_date=2021-09-01&categories=Promotions&categories=Survey&limit=500&aggregated_by=day

And I found the same problem mentioned in below issue had been fixed in python client
sendgrid/sendgrid-python#186.