PlayFramework Post Requests in Frisby.js

How do I get the right JSON body for Frisby.js tests when I connect to a play framework API?

For example, normally I call jquery ajax with dataType, contentType and JSON.stringify on my json object:

this.doAjax('/addsignature', success, error, JSON.stringify({ name: "app3", modulus: "mymodulus", exponent: "myexp"}), 'POST', 'json', 'application/json');

My play request().body() is:

DefaultRequestBody(None,None,None,Some({"name":"app3","modulus":"mymodulus","exponent":"myexp"}),None,None,false)

Then I can get it asJson().

In frisby I can't seem to do this combination of things:

frisby.create('AddSig')    
        .post(addSigURL(),
            JSON.stringify({ 
                name: 'app3', 
                modulus: 'mymodulus', 
                exponent: 'myexp'}),
            {json: true},
            { headers: { 'Content-Type': 'application/json' }}
        )        
        .addHeader(utils.AUTH_HEADER, utils.getAuth(userId, userEmail, userCommonName))        
        .inspectBody()
        .expectStatus(200)
        .afterJSON(function(jsonResponse) {
            checkSignature();
        })
        .toss()

I get:

DefaultRequestBody(Some(Map()),None,None,None,None,None,false)

If I remove JSON.stringify I get an invalid JSON:

DefaultRequestBody(None,None,None,Some("{\"name\":\"app3\",\"modulus\":\"mymodulus\",\"exponent\":\"myexp\"}"),None,None,false)

Which .asJson() doesn't work for.