How about this.. Specify query parameters and body..

    public HttpRequestData HttpRequestDataSetup(Dictionary<String, StringValues> query, string body)
    {
         var queryItems = query.Aggregate(new NameValueCollection(),
            (seed, current) => {
                seed.Add(current.Key, current.Value);
                return seed;
            });

        var context = new Mock<FunctionContext>();
        var request = new Mock<HttpRequestData>(context.Object);
        var memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(body));
        request.Setup(x => x.Body).Returns(memoryStream);
        request.Setup(x => x.Query).Returns(queryItems);

        return request.Object;
    }